It is a civil penalty and I don't think many of them are really going to be handed out. The guy in San Diego was told he had to leave and then told he was could be fined by the same guy. That isn't going to stand up in court.
Personally, I think if you are going to go to the airport these days you need to have your decisions made before you get there. Refusing both screenings is a pretty painful and stupid option. If you won't submit to either then don't fly.
I agree that's the rationale, I just don't know if it is lawful.
You cannot be detained/arrested for a civil violation. You can only be detained for probable cause of a criminal violation. If deciding that you need to go to the restroom because you have a sudden bout of diarrhea after entering a checkpoint is probable cause, then I suppose just about anything can be probable cause and the law means nothing.
Welcome to the police state.