Other mistake: When all the characters (Dracula, Frank, Murray, Wayne, and Griffin) all go to one side, the weight should make the tower fall down, but it didn't. However, after Dracula leaves and Frank goes to the other side, it happens to fall on his side.
Other mistake: If Bela can smell the humans from the castle during Denny's party, Vlad could easily smell Johnny as a human.
Other mistake: It is impossible for Mavis to fly from Usa to Transylvania in just few minutes and get to the castle at the same time that Drac and the others.
Answer: It does not matter that Dennis was not a member of the camp. Dracula dropped a child (Dennis) from a tower, just like in real life, that would be seen as completely inappropriate and unacceptable.
Rassdyt