It's not that there are a certain number of rooms available for the promotion. Try to follow me here ...
For each room category in each resort, they have a target occupancy rate in mind. If occupancy for any given date is ABOVE that target rate, then Free Dining isn't even offered to begin with. If the occupancy rate is below that target rate, they offer Free Dining, but only until the occupancy rate reaches/exceeds that target rate, and then it disappears.
Now, let's say the target ocupancy rate for, say, CBR preferred location rooms is 80% (I'm just making up the percentage here - Disney doesn't exactly e-mail me their super secret figures

). We know that CBR has 384 preferred location rooms. 80% of 384 is 307.2. So following this example, let's say that on the day before Free Dining is announced, CBR has 300 preferred location rooms booked for a given day. That means that when Free Dining reservations are open, CBR's preferred location rooms will be available for that promotion (for a given date). But as soon as they book 8 more rooms, they'll have 308 rooms booked, which is more than 307.2, and thus the next person who tries to book Free Dining in a CBR preferred location room for thatdate will be told Free Dining isn't available.
Continuing this example ... right now CBR is teetering at 308 booked preferred locaiton rooms, just over the 307.2 threshold for Free Dining. Then let's say at 3:00 p.m. someone calls to cancel one of those rooms, so now they only have 307 booked, which is less than 307.2, which means that Free Dining is open again!!! But only for one lucky person who happens to make a reservation while it's still available; once that next person takes a preferred location room for that date, Free Dining isn't available again.
Now, to address your question/concern, remember that 307.2 rooms figure, the target occupancy rate in this silly little example. Well, let's say that 370 of CBR's 384 preferred location rooms are booked. 370 is WAAAAAY over 307.2, so of course they aren't offering Free Dining anymore.
If they have 370 rooms booked and 10 people cancel their Free Dining reservations, it's not like you can take their place! There are still 360 rooms taken, which is still way over the threshold for offering Free Dining.
So there's no "unfairness" going on, once you understand how it works.
Let's say you own a fruit and vegetable store. Let's say you bought 500 pounds of red apples and 500 pounds of green apples. You paid the same for all of them and charge the same for all of them. Well, in a week you sell 475 pounds of red apples, but only 100 pounds of green apples. Your red apples are selling like hot cakes, but your green apples might rot in your store and you'll lose your money. What do you do??? You lower the price of the green apples, that's what you do! You lower the price to make them more attractive to buyers. Even if you make less money on them, you'll still get SOME money back on them before they go bad.
Same thing with rooms at Disney ... they don't need to offer free promotions on rooms that are selling well. For rooms that aren't selling well, they lower the price (or in this case offer something else for free ... same thing!) to try to make some money back. To a hotelier, an empty room is just like rotting fruit to a store owner -- they both represent lost money.
Does that help?