You're applying graffiti logic to a piece of cultural heritage because it happens to be a graffiti. That spirit is at the core of modern graffiti as an ephemeral medium by design - which is often why you'll find muralists tacitly tolerating the occasional bombing. However I think this particular piece of art had crossed into cultural heritage some time ago, and as such was a real dick move. Even using modern graffiti interpretation of the tenet, there are some pieces you just don't cross.