This has been my experience:

If a mural has been up for a few weeks and it's a non-comissioned piece, it's free game. You use better judgment though obviously. The consequence of being an ass is your own artwork being attacked. Petty squabbles do happen, but most can be avoided with a bit of courtesy. Part of the fun is the fleetingness...and also sometimes a bit of sporting griefing. When vandals vandalize your vandalization you cant get too mad unless you completely lack self awareness.

If an area is so crowded out people have to heavily compete over the same spot, chances are they'll naturally disperse and start hitting other non sanctioned areas, which is what we'd want to prevent. So I guess the goal for sanctioned walls is to provide enough vertical real estate to match demand, since only a fraction of the population wants to scribble on walls like toddlers. If youre serious about the hobby, it's 10-20 bucks per can of good paint. IMHO it's sort of a self limiting activity as long as it doesnt get too popular.

My favorite is when someone gets the bright idea to try free advertising a business on these walls.