[In BOS they have a sign before security saying that suit jackets and sport coats can stay on]

Exactly! The signs clearly state that suit jackets are not required, and I see them in just about every airport. As a matter of fact, I don't recall NOT seeing that sign. They're always there with the "in, out, off" signs as you make your way through the line. I guess TSAs use their own judgement when it comes to them, despite it being clearly posted. I have never been asked to remove my suit jacket, so I was shocked to have to remove my sweater.