Your argument hinges on the incorrect assumption that if they can't box or engage in MMA, that there is no other way to escape their social situation.  This is factually incorrect.

There are a plethora of programs that exist that arguably provide a much brighter outlook for these kids.  Study programs, work programs.  Programs like JROTC.  Hell, there are even far safer sports.  Basketball, baseball, soccer.  All of which teach dedication and discipline, and in the case of baseball, can even teach our kids some higher education subjects like statistics.

And to address your other point: It's not a choice when the person you think is going to help you escape your life only offers you one alternative, and doesn't bother to tell you of the others.  I'd wager that the majority of these kids who grow up to become boxers were told by their coaches that boxing was literally the only way to succeed in life.  Which, let's be honest, is a blatant lie.