Because it makes it appear like MyVMK is actually Disney.
As it stands, it is not clear enough that MyVMK is a fan-operated game. Clarifying that doesn't mean that the game isn't a violation of copyright (it is definitely a violation), but it does make Disney less liable to go after the game. Reason is, anything that goes awry in MyVMK could be attributed to Disney, and it could negatively affect Disney's image. If someone comes on this game, sees Frozen posters without disclaimer that it's not Disney's game, and then sees some questionable behavior, the person is going to think Disney can't moderate a game effectively. At that point, Disney could take action.
It's all a stretch, but it's important to take precautions for the worst-case scenario. Disney has left this game alone, because a cease and desist letter isn't worth the backlash they would receive if they killed VMK again. If MyVMK starts to make Disney look bad, then taking it down is the lesser of two evils.
Point is, MyVMK needs to look like Disney has nothing to do with its operation.