If a stranger (carefully) picked the lock to your home, then proceeded to vacuum-clean your carpets, mop your floors, and wash your windows, would you not call that breaking and entering?
(I'm assuming here that the original advertisements were placed with the permission of the property owners. Otherwise the placing of the initial advertisements was vandalism, as is replacing them with something else.)
Well I flyer for my business and we typically put a single flyer in several locations, sticking to billboards or phone poles.
Whats really gross is when an ad agency uses a "street team" to carpet bomb every available surface.
I also provide a bulletin board for public use in my establishment. I regularly have to remove the posters of people who put up ten or more and cover up other peoples flyers.
I also remove the flyers that people put on my front windows, the bathroom mirrors and even tabletops.
So really I see this issue as exactly the same as managing a message board. Encourage good content and delete spam or your board becomes a ghetto with a lousy signal to noise ratio.
As far as the artists go, whether they are better or worse than the ads is a subjective decision. If anything they are at least creating a dialogue which is worth something.
(I'm assuming here that the original advertisements were placed with the permission of the property owners. Otherwise the placing of the initial advertisements was vandalism, as is replacing them with something else.)