During the movie, Charlie is mad at Scott for not telling him about the relationship with Charlie's principal. Charlie goes into this whole thing about saying that his life has become all about secrets since he can't tell anyone that his Dad has the best job of all being Santa Clause. Well, he could tell everyone his dad is santa though if he remembers back to the first movie when he told his class of 6 and 7 year olds that his dad was Santa, everyone thought he was crazy and shouldn't see Scott anymore.

Now why would someone be mad for keeping a secret like that if you know what would happen if you didn't keep the secret. Other than the few people who saw Scott at the end of the movie, everyone usually stops believing in Santa when they grow up so no one would have believed him anyways.