Straight up, its not their property, so they should respect it.
Reminds me of when I was downtown with the wife, drove downtown in her car (328 Xdrive) pre Z
Anyways, we come out of a resturant and see some guys chatting next to our car and one of the punks is sitting on the hood. I walk and I'm like "Seriously!?" And he's like, my bad, and gets up
The wife found that hot that I said something, led to a nice night
So far people seem to respect the Z and are more than careful around it, but I'm sure I'll run across someone who doesn't care about their car or possessions as much as I do.