If you really want to get into the minutia, the onsite guests are required to put down a deposit and pay in full 45 days in advance for a package. Think they put that money where is doesn't earn them more? I would also venture a bet that they get a cut of my airline costs when I book flights through them.
Just looking at the PIF 45 day window.....someone above said there were 30,000 rooms. If 85% of them are occupied at any given time, that is 25,000 rooms. I will remove 5,000 for DVC/Villas where a payment to Disney may not be made. So, 20,000 packages of some type (room only, magic your way (old name), dining, etc). If the average stay is 5 days, that is 4,000 packages booked/paid for per 45 day window (I think). If the average cost for a stay is $3,000 (no clue), Disney gets to use $12,000,000 for 45 days to make more money. Even if they only get 3% for that 45 days, that is an additional $45,000. While they may only make a little more than $10 per room in interest (assuming simple interest model) over the 45 day window, scaling that out against 4,000 packages can't hurt the bottom line. If I am thinking about this correctly, the $45,000 would be multiplied by 365 days since a new 45 window pops up each day for another 4,000 rooms/packages. That would be over $16,000,000 earned from other people's money.
Add in the $200 per person deposit sometimes paid 400+ days in advance.......it goes on and on.