You didn't have to watch the game to know that he was playing with a previous injury. Remember he left the Falcon's game early in the 2nd half and didn't play against the Browns.
I'd guess that at least before Dallas he had time to get the right medical procedures and tests done to determine that he could indeed attempt to play.
The NFL is a business, it is not intercollegiate athletics. The players know it. They are in many cases paid more for a single game than most people make a year. There is a reason they risk their bodies. You and I may not agree, but they aren't being forced to play football. Like the little girl in the State Farm commercial says to Aaron Rogers in regards to being an NFL quarterback, "That isn't a job".