In real life, you can see something like that, and then walk around until you see the sign and have better context, and be less dismissive or confused. When all you have is a photo, that's less easy.
Oh I see, you're saying that he was an artist who's art was MISUNDERSTOOD! Well, that changes everything: In other cases, people understand 100% what artists are trying to do. No performance artist in their right mind can be expected to take into account that others might misunderstand his/her performance.
</sarcasm>
No, they (and apparently you) think that forming harsh judgment about the person based on a few superficial details, without any regard for the attempt at art whatsoever, is somehow okay. That the person is attempting art is just being used as an excuse for indefensible behavior.
Hell, the author would be better treated if he dressed up as a clown and made balloon animals.