To understand why graffiti is so vilified by the powers that as mere "vandalism" one must know what vandalism actually is. Painter Gustave Courbet (a famous vandal himself,) considered it to be the "destruction of monuments symbolizing war and conquest," a perhaps romanticised view I find difficult to agree with; since when did a bunch of chavs ever justify overtly the arson, for example of a bus shelter in terms of the shelter's socio-economic significance? I've yet to hear one. A more apt descriptor is that of vandalism being the expression of an alternate culture, in this case the "criminal" one.
Vandalism is executed against another's property in any case, and this is to my mind the root of the reason as to why it is considered unacceptable in a capitalistic society such as ours: it destroys the link between being a well-adjusted, functional person's exploits at work, and his material wealth. Tags on cars make them worthless, and consequently the hours spent at work tallying up the monies requisite to buying said car are no longer of worth to the worker. The tag has transubstantiated the work from a point where it can be used to buy things, where it is essentially useless. Needless to say, for the worker, this is upsetting. His car is ruined, and he's wasted his time doing reports on how John Doe broke his legs while sodomising a large black man on a non-regulation surface for the purposes of an insurance claim. The worker thus makes the link that vandalism annuls his exploits, and therefore must be in in opposition to his goals; much like an enemy.