Lord, this topic again?
My feeling is that yes, it's best to be courteous, but kiosk space is a limited resource. If someone covers your posters, the productive response is to put up more posters. It's a sales game, friends. If you really think you can change people's behavior by appealing to courtesy and fairness, you shouldn't be in the music business. Time musicians spend angsting about State Street posters is time far better spent on the phone bugging club owners and record labels.