It's actually even simpler than that. The airplane isn't just a "private business, and you shouldn't mess with their space". They're protected and empowered by broadly ratified conventions (which includes virtually every country in the word), starting with the Tokyo Convention:
> The convention [...] recognises certain powers and immunities of the aircraft commander who on international flights may restrain any person(s) he has reasonable cause to believe is committing or is about to commit an offence liable to interfere with the safety of persons or property on board or who is jeopardising good order and discipline.
And this is an offence? Or does he actually believe they're about to commmit a crime?
Is there some reasonable test? Could he do this for a band t shirt
You're being unreasonable. Think about what you're saying. It's the equivalent of "You can't wear that shirt, someone might get offended and punch you in the face." We don't act like this in society. You arrest the person who throws the punch, not the person wearing the shirt. Just the same way you don't arrest a woman for wearing something slutty, you arrest the person who sexually assaulted them. This is the definition of victim blaming. It doesn't matter if the victim is increasing their odds of being victim (unless they are actively seeking out and attempting to become a victim).
Be reasonable. Punish the person who is actually committing the offense. Don't punish someone because of some imaginary offense.