I would never even consider wearing a post MP shirt to a signing. But at the same time, I would've had to actually go to a post-MP show to get one.
Also, musicians live their whole lives creating some sort of drama through their music (prog music generally tends more towards telling a story beyond "let's drink and f*ck"). It shouldn't be any surprise when, as a matter of course, life imitates art and drama follows them into real life. If I were to recount all the times I've seen it in local bands in all these years, not to even MENTION all the times everybody sees it with national acts, I'd be posting non-stop for the rest of the week. Even the most friendly, fan-oriented, and likeable musician that you know, has probably had their moments.
Fan should've made a better choice of a shirt, he should've mellowed out and let it go. I find that the world often makes more sense when I just assume that everybody is an asshole and then let myself be pleasantly surprised when they show me otherwise.