They don't enforce it like that. Players swing all the time. Connect and the flags fly... Though I did witness 3 or 4 occasions in different games yesterday where punches were thrown and nobody was ejected. A couple of non-calls and a couple of personal fouls. But no other ejections after a swing, connected or not.
On another note, Asante Samuels should have been tossed after his blatant helmet to helmet hit on the Giant receiver. But I wouldn't have tossed him for the hit necessarily, I would have tossed him for his pimp strut afterward. If I were king that would have been his last play in the NFL.
Actually they do enforce it like that, and no players don’t take swings all the time. I watched all day yesterday and Seymour’s was the only I saw thrown. The league is very serious about no punches. Pushing and shoving they’ll tolerate to a degree, but when a punch flies it’s an automatic ejection, and the players know that and are usually smart enough to not do it.