All internet boards, chat rooms, specially those NOT owned by large entities have total control over everything on their websites, if they find the need. Censoring in this liberal website, liberal meaning with the rules in general, I find that it would be unlikely.
As a moderator in a small chat room of maybe 30 people or so for the last 8 years, they all know the rules, everything is logged, whispers and general chat. As a rule, no one is ever censored, unless they become extremely abusive to another chatter bringing their families into a dispute yada yada and yada, then they are warned,then booted or banned if it continues.
I personally wouldn't have yelled 'censorship' without first doing all your homework first gathering information in hopes of proving it one way or another. I would guess there is NO censorship in USMB, only their rules and guidelines to adhere to.