The difference is, your inbox is yours. Those walls belong to someone, and if they want to turn a profit renting the space to advertisers, that's their business and no-one else's.
edit: Questions of legality aside, I mean.
edit: Questions of legality aside, I mean.
Again, the original postings weren't allowed (like spam to my inbox).
There are some who argue that billboards are like noise pollution, but with your visual instead of auditory space. I think that is partially correct, and so I don't mind this kind of corrective action.
Note that this kind of of public art display is exactly what makes NYC a vibrant place to live and other areas really boring. That kind of vibrancy definitely has economic value.
If you are making money by breaking the law and annoying people then I have far less sympathy for them than if their legal private property where damaged. IMO, It falls under the category of destroying illegal campaign advertising on public land.