[geophile]

There is a difference between banning something, which is imposed; and shutting down, which involves no 3rd party.

Boxing: Not banned, but a lot less popular in the USA than it used to be. Maybe this is due to better understanding of the physical toll (brain damage, the occasional death resulting from pummeling in the ring), although there were other factors too. People going in know, or should know, the risks.

Skiing: If you want to risk your knees, go ahead. You know about the risks.

Football: The risks are finally becoming known, but the NFL seems to have known the risks for years, and sought to suppress this information from spreading. The other physical risks are known, and participants make their choices, as in skiing. But the NFL has tried to fool participants about the extremely serious consequences, much like the tobacco companies did with the health risks of cigarettes. Coaches and recruiters -- even in high school -- know the risks and turn a blind eye. How an informed parent can let a child participate in football now is beyond me.