Exactly

Disney may market the FP system as "saving your place in line" but what they are really doing is getting you out of line to spend money someplace else.
The idea is really genius. I really wish the parks around here would adopt something similar.
My husband spends all our time in WDW telling me exactly this. "they'd rather have you walking around spending money on merchandise, food, etc than standing in line with your wallet in your pocket, THATS why there are fast passes, not for our conveniences...just so we will spend more money" And he's right!
It is truly awesome that there are so many people on DisBoards with strong opinions on so many topics. However, sometimes the opinions seem to be formulated without much basis.
Do I think for a minute that Disney ever makes a decision without the good of the Walt Disney Company in mind? NO. However, to say that Disney has implemented a "guest experience improvement" idea, like the FP, SOLELY to increase it's bottom line and not for any other reason is a bit.....well, ignorant??? (sorry)
First off, if you wanted to break down the total number of people in any given park that would be standing in any line if there were no such thing as the FP vs. the number of people that would be in line with the FP system, I don't know that the number would be any less. If there is a one-hour wait at TSM and 15 minute wait in the FP line, do you think there would be much more than a 1 hour and 15 minute wait if there were no FP?
When the typical guest exits a ride that they just used a FP on (say BTM), I'm guessing that they would tend to head for another ride, perhaps one that doesn't use the FP (say POTC). In the meantime, the would likely be grabbing another FP (for say SplMtn). I don't think they would say, wow, that only took 10 minutes, we saved 40 minutes by not standing in the standby line, lets go eat or shop. The FP system (when used efficiently) allows and educated guest to ride more rides more times and thus increases their overall experience in the park, thus bringing them back to the park more often.
If a guest gets a FP (for say Space Mtn) and the "use time" of the FP is not for an hour, I doubt most guests will say, "let's go spend more money while we are waiting" or "hey kids let's go into the gift shop for 50 minutes, I've got a wad of cash burning a hole in my pocket". It's more likely that they would hit the restroom, ride the People Mover and maybe stand in line for AO.
Most people, I would guess, come to the park with a certain amount of money that they are willing to spend on souvenirs and food (and let's face it, with FD, you can only eat so much). A FP is not going to make them spend more money.
I will agree, that giving the guest a better overall experience and an opportunity to utilized THEIR time in a more efficient and self-serving manner will give them a much more pleasant experience and offer a much higher chance of them coming back. By getting them to come back, yes, Disney makes more money.
But, I highly doubt that the FP was put into place to get guests to spend more on food and merchandise with THE CONVENIENCE OF THE GUEST BEING A SIDE EFFECT.