I asked this question and was told that boxers are, in essence, private contractors and the NFL is a company. If the company withholds evidence and encourages people to play in spite of clear science that determines it's dangerous to your health, you can sue the company. If a private contractor suffers the same injury due to sport, who can he sue? Everybody knows that getting punched in the face for a living is likely to cause significant long-term damage.

What's interesting is how many parents are taking their kids out of football to have them play soccer while not realizing that soccer players suffer a lot of concussions as well. I'm amazed to know that Rugby is apparently safer than soccer and tackle football.