Humans are artistic people. Especially when it comes to community and show casing it. Through out history people have done everything they can to do art wherever they have been. Including what is now called tagging and graffiti.
Its funny cause we don't consider it graffiti when its a archeological find from century ago. Some of the things that helped paint a picture of every day life. Did you know there is a viking runestone in Oklahoma? Sure sucks that ancient viking left that and dirty up the place huh?
And you'll say "but this is different" how? Do you think random viking guy knew his random ass stuff would be consider a archeological gold mine? We never know what will be important like that in the future. Its literally all the same stuff the only difference is the when and how.
Tagging is not something that is going to go away. moralizing it and policing it is not going to work. So its time to re examine it and figure when is it actually a problem and how to channel it in the correct ways.