You do understand that every social and cultural more that dictates modern law and social etiquette is the result of someone's projected expectation, right? There's not anything inherently good or bad about anything we do, but centuries ago some folks in positions of influence decided to assign those values to certain actions and behaviors, based largely on their personal religious values, and now here we are. 

With that aside, you're being something of an unreasonable pedant. No, the game doesn't start at the anthem, but they're on the field in uniform representing their teams, the team's owners, and the NFL at large, so it's not unreasonable to expect them to conduct themselves with some modicum of good graces even at that point.