If you are arrested, the promoters will refund the purchase price of the shirt. T-shirts are not the only item that might get you in trouble with the Feds. If you wish to have private communications over online services or wireless communications, you might have non-exempt encryption software on your laptop computer. But if you take that computer on an international flight without a temporary export license, you risk arrest as an arms dealer. Fortunately, bills have been recently intoduced in Congress to loosen this law.
Cryptographic software is essential to development of commerce in information.
If the U.S. restrains free development of such software, U.S. technology companies will be faced with a state-induced disadvantage. The World Trade Center bombers were caught when they tried to recover a deposit on the rental van they blew up along with the building. And the government has devices that can read the contents of your computer screen from outside your office.