[Butch Cassidy Slept Here]

There is no credible "safety" argument the TSA or the airlines can mount to support denial of boarding because somebody is wearing a t-shirt, or wrote something on a boarding pass, they didn't like. Sorry, but contrary to the assertions of some TSA Officers, the First Amendent DOES apply on airport property and aboard US registered aircraft. The airlines, might, be able to make "defacement" of a boarding pass a reason for denial of boarding--provided such a rule was incorporated into a Contract of Carriage. However, if the airline starts distinguishing between "correct" defacement, and "incorrect" defacement, then they're headed for trouble.

The IRS has tried, unsuccessfully, prosecuting people who have put nasty notes on their income tax forms.