Football is a contact sport, and it can be violent sometimes. Players should be able to hit each other, especially in the head. Fans want to see action when they watch football. If players can't express themselves naturally, it will be too boring to watch. No one gets seriously hurt, anyway.
Most NFL players do not last more than 4 or 5 seasons. Assuming they started playing as soon as they finished university, Professional players are done before they reach 30 years old. This means that players will be spending at least the next 30-40 years doing something other than professional football. these players need to be protected because their lives depend on it.