People are really stupid. Even if you had never heard of TPB you have to know that shirt is a joke right? I see references I don't get all the time, but I can always tell when they are meant to be humorous. In fact, if I don't get a reference such as that shirt, I AUTOMATICALLY assume it's meant to be humorous and forget about it.
One other thing. These people are especially stupid because it seems like it was the actual shirt itself that made them nervous, not the guy wearing it. Their "solution" was to eliminate the shirt itself as though that really fixes anything. If anything the guy who is wearing the scary shirt is what you have to worry about, something which can't possibly be helped by antagonizing him during the flight. Scary shirt may be gone, but scary dude who was wearing it hasn't gone anywhere, and now he's pissed off to boot.
My conclusion is that they weren't really scared at all. They completely understood that the shirt was a joke, but were vaguely annoyed by someone having the nerve to wear a reference they didn't understand. They were exercising their ability to be busybody anal fucksticks, and getting off on the idea of forcing someone to do something over an issue so small as to not normally be worth mentioning. I hate these people and wish they would die.