About 5 years I had a very bad private screening backroom experience with the TSA. It still haunts me every time I fly. I made inquiries into the legality of what happened. What I was told is that in a post 9-11 world you give up all normal rights once you step into an airport. If the FA says you were speaking in a threatening manner you may very might find yourself in the company of airport police. Ultimately they’d probably let you go but not after an ordeal.

You have no “rights” when flying.

In this case I would have just said ok when the FA said no coffee.