To my knowledge, every hotel’s rates are based on double occupancy. They generally have two beds because one is for the adults and one is for the kids (or juniors). The majority of the time it is a family that makes reservations, rather than four adults. Not to say that it doesn’t happen, of course, but rates are based like that because generally two adults are the norm.
Here are some of my speculations:
Hotels, like any other business, is in business to make money. They also need to protect themselves a little bit. I have literally seen 7 people walk out of a room at a Holiday Inn once. They were all looking around hoping that nobody saw them. Obviously one person made a reservation and then had all his friends and family sneak in.
Housekeeping: The more people in a room, the more mess they will make. I know that housekeeping is on a set wage, but it still takes longer to clean a really messy room.
Products and linen: The more people in a room, the more products (shampoo, soap, etc.) have to be replaced. Also, the more towels and linen are used, and have to be replaced.
Like daisyd said, it’s still a LOT cheaper than booking two rooms ($15.00-$30.00 compared to another room charge).
I don’t believe that Disney is attempting to “milk” anybody out of extra dollars in the amount of $15.00/person.