Fine. Lets assume for the sake of argument that said transportation company is owned privately, and any cost of clean up is forwarded onto the consumer. Even if the financial end of things are absorbed into society, the act in and of itself is not acceptable.
If we were to universalize the maxim of the act as laid out by the categorical imperitve: first formulation, we would be blatently odvious that such behavior made acceptable would not benefit the society at large. Rephrased, would it be acceptable for everyone to run around tagging all things public and priviate, including YOUR car or YOUR house that YOU bought with YOUR money. Christ, just because it doesnt cost McDonalds too much if someone steals a cheeseburger doesnt make the act of stealing right...less wrong maybe, but still morally reprehensable.
So come on people, taggers and supporters alike, lets find a more productive way to expose our talents because graffiti isnt really helping anybody.