There are several situations in which that argument does not work. For children and high school students for example, who are not able to make a knowledgeable decision to assume such risk. Another one would be in the case of a high school or youth league-- can that league assume all risks and consequences and make decisions for minors? I played football and love the sport, but would not take the risks or allow my children to do so. Talking to other guys who have played, several agree-- a few of us (myself included remember at least one concussion but findings now say repeated shock without a concussion also causes CTE).