Well, because it's up to the park owners/management to decide what types of tickets they sell, when they let people in etc.? There's no particular right given to a daily ticket buyer that the parks will be open to them exclusively for xx number of hours. Nor is there any guarantee of a particular crowd level etc for it will be closed off to new entries when it reaches capacity. Also, Disney has said they pay for that time since they let them in so I don't understand your premise. Just because it was something different before doesn't mean that they can't change the price, hours etc. They do that all the time.
And just look at some of the hours they release now. If the party wasn't going on I wouldn't be surprised that they'd close the park at 8PM or even 7PM if they thought they could.