Are you new here (to this planet, I mean) that you think law determines truth? People get fired for illegal reasons all the time, why are you so convinced that something so "cartoonishly evil" could not happen? A rule just existing in a book somewhere isn't some mystical rune that curses every business owner to act within its limits.

He's a teenager at the time. What's he gonna do, sue them? Call the police, drop a complaint form to the NLRB? Lol.

Even assuming he (and the business owner) knew his rights at the time, he has no way to personally enforce them without some strong organization (union) sticking its neck out to back him. In most cases, especially those under at-will employment, the employer doesn't even have to give a reason to fire someone, which is already a mountain of plausible deniability in the employer's favor.

Of course without that, there's all sorts of bullshit reasons the business owner can make up; just as you've done here; to "prove" their side, that will confirm the same biases you've so clearly put on display for us without even knowing the guy.