Strictly speaking, they aren't publicizing any opinions while on duty. They explain what it's about outside of the game, and on actual game days it's just a nonverbal gesture.
Is there even any requirement stating they have to stand for the anthem? If there is, you know it's just a technicality and that's not the real reason you disapprove of it. I'm sure there are lots of minor things people violate about their jobs regularly that you don't bat an eye at. They can try to make them stand but you and I both know that these guys don't earn their checks for their ability to stand during a song.
Thats just it, they arent required by the league to stand. Its encouraged but they dont have to so they arent violating any codes of conduct or anything by kneeling. If the NFL feels that theyre taking enough of a hit from a financial standpoint to actually try to make it mandatory theyll have to try when they negotiate a new deal with the players.
But the NFLPA could sue on the grounds of it violating the first amendment and thus be unconstitutional. ---
Fall down, go boom... Life is like a box of chocolates. Most of it is crap.