I agree that most of the graffiti is dreadful and disrespectful.
However,I saw some fabulous graffiti art walking next to a long building in Chueca- I can't remember the street!
Some years ago in the States spray paint was locked up in stores and therefore not as easy to buy.In my neighborhood there's a phone number to call to report graffiti and it is painted over very quickly.I believe the fear is that the graffiti is done by gang members!
The trains in New York used to be dreadful but they have that under control nowadays.
The saddest thing for me is when you try to take a photo of some historic site and you can't find an angle to avoid the graffiti.