You are still operating under the assumption that the FAMs are there to battle terrorist hijackers. But stop and think about the way that the Air Marshal program was introduced. If the FAMs were supposed to actually defend the aircraft against terrorist hijackings, what purpose would it serve to announce loudly and publicly that FAMs will be operating undercover on flights? Maybe to keep the terrorists' odds of success from being too low for a "sporting fight"?

I still believe that the FAMs are there to make you (and me) feel safer flying. In that case, it would make sense to announce that they will be there (in fact, it wouldn't do anything at all to make us feel safer if we didn't know they were there). It would also make sense to have them be somewhat discreet, but not too discreet. It should be possible for an intelligent observer to spot them.