According to reports, the shirt made a lot of people on the flight "uncomfortable". Thats the standard and the reason why the case will get tossed. If a good number of passengers on a flight feel threatened, unsafe or even just plain old uncomfortable by the actions of another passenger, that passenger causing the problem will get booted from the flight.
Thats the SOP of all carriers. Next time there is a problem with your flight and someone is acting up, take notice of the flight crew. Ill guarantee they will asking all the other passengers sitting around the person causing the problem if that passenger is making them feel uncomfortable or unsafe. If a good number of people raise their hands, Ill further guarantee the next member of the airline staff that comes to see the person causing the issue WONT be offering coffee and tea...they will be asking that person is s/he would like to to be removed from the aircraft under their own power or by force.