According to TMZ (via a source close to the couple) Mariah put a gag order on him so tight it will cost him some serious financial penalties if he speaks about her. Apparently, the confidentiality agreement is completely one-sided.
While Nick can't speak about their divorce at all, Mariah has the right to announce the split on her terms. So his announcement this week probably served as a generous freebie on her behalf. I can see this getting nasty already. Though, how touching.