I say: good. Drivers as a general category are incredibly hard to keep in order. There's something about cars that makes many people lose their sanity.
I'm not a lawyer and definitely not a lawyer in "almost everywhere", but it is surely illegal in almost everywhere with western-style property rights? Especially if it left residue after the fact. On the other hand, who is going to take this to court? These are largely one-off tourists who won't have a vested interest in the matter after tomorrow.
People are very possessive about the state of their car's exterior, imagine someone sticking one of these on a high end car.
Also, it sounds to me the area is not actually a "no parking" area - it is at least conceivable the court's interpretation would be different if the claim were true. Residents are painting the curbs themselves.
Not a lawyer either, what I've read about street art in Germany is basically: if it leaves residue (physical changes your object) = property damage, if it's easy to remove without a trace = no case.
Which is why a lot of street artists swapped the can for painted paper and water-soluble glue.
Or, related to cars: hard to remove stickers = property damage, some chalk on the tires or shaving foam on the hood = no case, deal with it.
In BC people have sued the police and won for "property damage" when the police put chalk on their tires, so I'm pretty sure the same would apply for this sticker.
Oh, they switched to harder-to-remove ones at some point, because douchebags tended to remove them and continue following their douchy ways of behaviour.
A hard-to-remove sticker that takes up a third of the windshield is a really great idea IMO; all the "Putinjugend's" issues aside, I think StopXam are doing a great work.
Quite often, recipients of those stickers totally deserve it. I recommend watching some of the videos from the channel 'vermontdevil linked to.
I thought few times about starting something similar in Poland but then I understood StopXam really works only because they're government-backed program, so if someone gets too angry about having half of their windshield covered in a "I don't respect people" stickers, the police will bring him to order quickly.
One of my very first deployments was to Germany, where we guarded a hospital. People like the colonel's wife would park in the ER drivethrough for up to an hour at a time. Yes, blocking emergency access for any ambulance that might need to come through. I got very butthurt about this and the old sargeant of the guard on duty smiled real big and opened up the bottom drawer of his desk, dug way back in there, and extracted a stack of 8.5x11", bright neon orange, "impossible to remove without fucking up the car window" stickers that said nasty things about parking like an asshole (in German and English). They were very aggressive stickers. But imagine my delight when he told me to stick them on the cars parking in the ER drivethrough. My only rule was that I couldn't put them over the driver's side windshield and block their vision forward. So I started slapping these fat motherfuckers on the driver's side window. Adults wept and threw tantrums when they saw these stickers. The kind of reaction you would expect out of someone that parks where the ambulance needs to go at a hospital when bringing in someone that might be dying.
There are times when these stickers are used for the forces of good, you know.
Edit: I still have about half of that stack of stickers at home. I still only ever use them for the forces of good. :)
A hard-to-remove sticker on the windshield for parking in the wrong place? Won't that guarantee that the car will be staying in that wrong place for a lot longer? It doesn't sound safe to drive with a sticker in the middle of your windscreen.
This is just plain destructive and counterproductive. Use something that's easily noticed but also easily removed.
Vandalism, basically.