It does take money out of the precious limited train budgets to clean them up, especially when they paint over windows which is a hazard in an emergency situation.

Its sorta like the broken window fallacy(not broken window policing), that doing something bad(ie breaking a window for a glass person and carpenter to replace) doesn't make a job have more of a purpose by giving them something to do, but actually hurt the budget and possibly cause positions to be lost.

edit: Dang, I'm not talking about the broken window theory of policing areas. I'm talking about those dudes who tag the sides of metro trains, all the way up to huge organized whole car paint jobs. Everyone loves trains till they got nicknames on em. All these downbears for defending the trains.