If this was a case of deliberate disrespect, then the kid got a lesson from someone who also wanted to make a statement, so no problem, we're all free to express ourselves.
Whatever happened to ..'your freedom ends where the other guy's nose begins'? A 39 yr old who lives with parents seriously injures a barely teenager for a disrespect shown by a majority of sports fans every day needs a bigger lesson than he thinks he gave. He taught a lesson alright, but about himself more than anything else.
Actions have consequences; whether they are right or wrong means nothing. Mind your manners in public places, and you don't have to keep winning Darwin Awards like some dumbass punk, is all. You have a problem with that? Think there are magic wands out there that you can wave around and avoid unpleasant reactions to your childish bullshit? Guess again, or just stay home if you don't know how to behave politely in public. Same with burning flags and the other neurotic kiddie crap for the Stuck In Da "60's tards; don't be surprised if one day you do that infantile crap in front of the wrong person and regret it. Be as free as you want, feel free to offend everyone on the planet. Don't run around whining about the results.