Channel Awesome had a few incidents where a corporation threatened to sue if they didn't take their review or parody down, and they pretty much had to shrug and take it down until they could convince the corporations not to sue, even though legally they were in the clear (usually the argument "this gives you exposure and free advertising" was involved).
Again, good lawyers cost money. Satire is protected, but a lot of corporations are very zealous, either because they have to be to maintain their trademarks or because they want to suppress negative comments on their product. As a small fry without a lot of money, you're really at their whim if they seriously threaten to sue because even if victory is near assured, the legal fees may outweigh the benefit gained from winning (or put your business under before you win at all).
ETA: And yeah, the T-shirt is murky territory. You have to be very careful about Trademarks since I think they have somewhat complex rules, talking to a lawyer isn't cheap, but it may be worth it depending on how big of a behemoth the owner of the concert is.
Last edited by Jragon; Today at 08:28 AM.