yes, it's expected that you'll never do or say anything bad about past jobs(/companies/environments/people).
But other side of this status quo is enabling all the assholes to be assholes and just helping them to continue behaving/acting like they do.
This type of withholding of information prevents others from making informed choices.
"Don't burn your bridges" implies that these are bridges worth preserving--sometimes they aren't, and you're doing others a favor by letting them know.
that's a bit offtopic though.