Actually, they didn't donate that amount. If they want to calculate the costs they incur for individuals just walking into the park, then sure, they might have donated that amount of money, but lets face it, getting a free ticket into a park is a minuscule part of what you spend when you go to Disney. Food, drinks, souvenirs, any other incremental charges (parking, etc). Then add to it the fact that the vast majority of people who don't live where a park is located will be getting hotels (perhaps at one of Disney's properties) and will stay for several days, Disney is basically adding a million customers who may or may have not been planning a trip to Disney already. They are not losing a dime on this and will gain quite a bit of money.
Don't get me wrong, I am not being critical of them-getting people to volunteer is a good thing, but given Disney will probably get some massive tax write off plus generate tons of new revenue from those folks getting their free ticket while they blow tons more money at their parks, I am guessing they will come out way ahead for this...plus engender good will with a lot of people.