How do you know that those empty sites are being used in the way OP is asking about (i.e. paying to get the benefits but not living there?).
So did the CM say there wasn't someone physically sleeping there last night but no reason why, or did they specifically say they knew that someone had booked it just for the benefits?
If the CM said someone wasn't sleeping there last night, then it is still very possible that they got held up in traffic, had to cancel last minute, or something else. I doubt many people do as the OP asked about, and there are plenty of noshows.
On the other hand, if the CM said someone booked it just for the benefits, then I think that's telling you that Disney doesn't have a problem with this strategy. They're condoning it even, by not doing something about it. If they felt it was against their rules somewhere, they certainly could tell the person they had to give up their reservation if they weren't going to use it. Or if they couldn't find the person to evict them, they could certainly turn off their KTTW cards to force them to have to go to Customer Service for help, or lose their dining plan benefits, etc. But, my guess is IF, and that's a big IF, they knew or cared to know, they wouldn't care, so long as the payments had been made. As another poster pointed out, from a business standpoint, they don't care......they have a paid for room. They were paid $38 a night for each adult's DDP.....that means that the person is eating inside Disney instead of off property or at their rented condo/home. Most people who stay offsite eat several meals not at Disney, so Disney would lose that money, but in this case, they collected for the DDP....if you don't eat on site and use your credits you don't get a refund, so again, they won't care if you don't eat....they already got paid.
So, where is the downside for Disney? They still collect money for a room/site, they get paid for DDP...they don't care if there are a few extra bodies in the park for EMH, and parking is a negligble loss anyway, as it's actually factored into the room cost...so those who don't have a car are paying for it anyway (though using the bus probably equals the cost).
The only downside is for other guests who had hoped to stay there, but can't because it's booked. Do you get mad at people who book a vacation 10 months out, make all their ADRs at the 180 days mark, possibly meaning you can't get the room you want nor the ADRs you want....and then last minute they have to cancel? Their cancellation didn't help you, because you already booked at a higher level of resort, or changed your dates, or couldn't go. Of course not....in fact I've seen many times people post they had to cancel their whole vacation and replies come pouring in with sympathy and condolences. Haven't yet see someone say "yippee, now I am going to call CRO and grab their room and ADRs before someone else does".
Disney is pretty savvy about what is happening on their property. One of the biggest flamings that I see is for people who openly admit that they use a Refillable Mug at either a different resort than they're staying....or an even heavier flaming offense, bring their last mug back on their next vacation. Obviously, if Disney really thought this was unacceptable, there are a zillion ways that they can make it nearly impossible to do......until the last year or two, they had different mugs for each resort, so it would have been easy for a CM to spot a "foreign" mug and politely tell the guest they can't use that here. But they don't. Instead, last year (or maybe the year before) they changed the mugs so that they are ALL identical....now the CM has no way to know what resort you paid for....or if you even did this stay or at all (do you have to show a room key to buy a mug???? or can someone eating at a resort food court living off site pay for one, they appear to be out in the open). If they really wanted to stop this, they'd simply change the design every month, or every quarter, or even every year, have CM's police for foreign mugs. But they don't, they actually made it easier to cheat the system. I imagine that the savings in having one design was worth it for them....do you really imagine that everyone in your party drinks $12 worth of DISNEY's COST of soda? Soda is the leading profit makers for most restaurants as it costs pennies per cup but they charge dollars! Not just Disney, but all restaurants.
Now...for hard ticket events, like Pirates and Princess (we went to those, so I know about them, but I'll venture to guess that Halloween and Christmas parties are the same)....they have found a way to prevent guests from cheating the system. The wristband color changes every night.....and it's not consistent enough to be able to say "Mondays are blue".....it's random enough to make it incredibly difficult for a guest wanting to cheat to have somehow accumulated every different color band to reuse. And...Disney knows these are resold on Ebay....but they don't do anything to stop those sales, there are people selling things like this, and
AAA parking passes, and even MYW tickets, and they have been for months (look at their feedback) or years without being stopped. Do you really think Disney doesn't know. By the time you bought enough color bands to be sure you can find the right one for a particular night, you've probably paid for a ticket, lol. Might be an interesting poll to find out how many different colors they use.
Disney as a business knows that some people are going to find ways to cheat them.....it's an impossible goal to try to stop them all, so instead they pick and choose which they are actually losing money on, like the parties, and find ways to stop those cheaters. The others, they chalk up to the cost of doing business. Same thing about not requiring a parent to prove a child's age.....not a difficult thing to require after all....but they don't. So lot's of people have children who are miraculously 2 years 10 months old even though they certainly appear pretty tall for that age, lol....or the 9 1/2 year old who also appears not only very tall but ahem, developed? My girls were only 3 months past "child" age when we went....I wish I could have paid for a child's AP instead of an adult, but I couldn't bring myself to do that, nor did I want to tell my child to lie if asked about their age (that would have been worse to me than my own lying!).
Disney obviously decided that people buying tickets and then sharing them was not a wise business plan, and put the finger scans in to prevent this....but you know what, I'll venture to guess that on any given day there are hundreds who get through it unscathed. Not because Disney is letting them, but because they discovered a loop hole that they could use....I can't begin to tell you how many times my AP would fail the fingerscan....why, I have no clue, but probably half the times I tried it they had to reset it.....oooo, a loophole...if you can hold your finger so it's not obvious, but it still fails....you MIGHT get your fingerscan reset too.....and if it's an "illegal" ticket and you get it reset you won. But, I wouldn't risk not getting in for a savings of even 90% of the ticket cost...but there are others willing to try it for a mere 10% savings, or anything in between.
Of course, the final say is what you are willing to do....OP apparently is at least considering doing this (and from looking at his few previous posts he's already stayed at FW so he knows the set up). It isn't a benefit that I would even want.....so I don't even have to wrestle with the "right/wrong" question. My gut says it's not quite fair....can't totally pinpoint why, just doesn't feel ok, so I probably wouldn't. But....legally it's not wrong, and Disney appears to know about it and not do anything, so it's not against any policy they have that they can use to prevent it....and believe me, they can easily make such a policy to stop it in the future if they want.
So, as with many gray areas of the law, it all comes down to letting your conscience be your guide...if it bothers you don't do it...if you think it's perfectly fine, do it....if you're somewhere in between, you'll have to wrestle yourself.