The airlines should just issue a dress code for flight. Then no one would be intimidated, offended, or fearful of a person's attire. This is what this world is coming to anyway. No freedom of expression. Let's let everyone wear what the ones in charge tell us to, so everyone can be happy. No one will complain, and all will be right with the world.
Banks frown on attire that hides a person's face, and even have a "dress code" posted at the door.
Restaurants state, "no shoes, no shirt, no service".
If the airlines believe that someone who wishes to do harm on the plane is going to advertise it on a T-shirt, then just have a dress code stating that it is not appropriate attire for flight and you will not be allowed to board.
I personally do not agree that this is the right thing to do, and I can't believe he was asked to remove his shirt. But, if this is what they want, as a private company they have the right to post it as a rule/dress code. But, they cannot pick and choose who to enforce it on, they'd better be consistent.