NV290 sums it up perfectly: it's vandalism and a crime. Look upon it this way: what if someone tagged your automobile or house? Some of these guys actually tag HO-scale cars and sell them at art shows... maybe they're raising money to buy more spray paint.
We've come a long way since Bozo Texino.
I was thinking about the graffiti problem and came up with a wild hare idea about how to combat the issue: Sanction tagging by actually offering up cars needing paint to taggers. Park cars at locations where they can have at it, with one condition- that being that all car data (reporting marks, instructions, etc) would be masked off. Think of it... the railroads would save money by not having to repaint cars, the biggest operational headache (the painting-over of car data) would be reduced/eliminated, and (eventually), the taggers may get bored with the legitimacy of it all and move on to other pursuits.
Ok, this is a bit tongue-in-cheek but, liability issues aside, it would probably be more effective than preventative measures being taken now.
Moderator's edit: Consecutive posts merged 3 December 2009 630AM CST