The two guys aren't blatantly wearing Mario and Luigi caps, they have different letters there I think. Costume colours are pretty obvious and they have mustaches, Bowser is just a guy with a mask and a really crappy costume, Peach doesn't look like Peach. She isn't even blonde.
I guess Nintendo could do something about it, but people doing cosplay sort of thing (with very accurate costumes) and getting paid would be more likely to get into legal trouble.