I would agree on that, but: the final outcome could still be real art despite the "artist" having asked for permission or just took the initiative and "created" on someone else's property. What if Picasso (or any other established artist) was a graffiti artist and filled the streets with murals at will? Wouldn't his creations still be art?

What I am trying to say is that we should be trying to cultivate respect and good manners. It is an indication of how educated people are. A talent is wasted if the artist acts like an animal within the society they belong to.

I just remembered this mural by @romanie, street mural, Ibiza 2016

And this is just an example of the graffitis in Thessaloniki (I found the photo on the internet, but I have walked past this building and it is very beautiful):