I suppose if you wear any shirt with some sort of slogan, or prominent in-yer-face brand name or logo, people assume you're wanting to make a loud statement, broadcast the fact that this is something you're into, or part of an insular club. They think it's there to be commented on.
A Maiden logo tee, like an AC/DC, Metallica, Motorhead, Priest or Slayer one, has that link with trendy celebrity culture too. A squeaky-clean starlet wants to dress up as a rebel, she appears with messy hair, smokes a cigarette, wears a leather jacket, ripped jeans/fishnets and a tee for one of these bands. Retro 'classic' band chic, maybe.
As with Mr.K, I've mainly had positive comments and conversations starting with 'I was at Donington in whatever year'. I'm wary about wearing a Maiden tee in the town I live in, though, parents are very, very touchy about what their kids see. Police and shop security are terrible for following anyone who doesn't fit the leafy suburbs stereotype, too.