Graffiti is an art of protest. It protests against a lot of things: lack of access to property ownership, lack of access to class privileges, poor use of space (both public and private), bad aesthetics and more. Since it is protest against so many things, the people being protested against don’t like it. In particular, owners of things don’t like it. The last thing they would ever do is call it “art.”
Like all art, it is a matter of taste. It is an aesthetic that most people don’t like, primarily because they feel like it is an attack against their culture. It is hoodlum culture, which is to say, in most people’s eyes, not culture at all. If you don’t like the idea that it is hoodlum culture, then look at the roots of the form: young men wandering around with nothing productive to do and no place in society.
I do not mean to disrespect graffiti by calling it an outcome of hoodlum culture. I think it can be beautiful. I think that its ethos of protest is relevant, although I do think most people don’t get it. I’m not even sure the artists get it.
In my town we have an anti-graffiti network that sponsors murals on all the blank walls all over town. This does tend to use space in a way that discourages graffiti. Graffiti tends to appear on poorly used surfaces. Abandoned surfaces. When those surfaces are painted on, they become used surfaces, and graffiti artists and taggers alike seem to respect that. So it does seem to me that, in general, graffitists have a code of conduct that is not entirely anti-social.
Anyway, given its roots and its meme of protest, it is not hard to understand why so many people find it offensive. In addition, it is a special aesthetic that not a lot of people have. You have to be on the edgy side of culture to like it, I think. You have to be a protester of some kind. Well, I’ve spent a lot of my life protesting this and that so it will hardly come as a surprise that I enjoy graffiti art. Indeed, I have a work by Keith Haring hanging in my kitchen. But then, maybe that’s just the white bread approved form of graffiti, and besides, it’s so eighties.