This is my view on this and not really a "rule" spoken or not.
The general public wears shirts from a competing park all the time (ie. wearing a Universal Studios Florida shirt at WDW). I consider this in bad taste. I actually plan on wearing my SFMM shirt at another Six Flags park or my BGW shirt at a Busch Gardens / SeaWorld park. I consider this free advertising for other places that the same company operates. I just got a Canobie Lake shirt. Since they are a small park and I don't believe that they are part of a company, I think that it's ok to wear it just not in New England. I usually get some kind of reaction to mine from another website, and that I can wear any place. The "Roller Coaster Junkie" one is a big hit; I have taken that to a lot of parks and many people want one.