Some places have bylaws on maximum lawn height and you can actually be fined for letting it go. That's how insane people are about lawns.

I mean that's probally overkill, that person was either OCD or was thinking he was doing them a favor. That sounds like a great way to have a pissed off neighbor and a potentially hostile neighborhood

I doubt that I could have demonstrated real harm, or even proved that he did it. I got back into town a week later and my brother, who had been watching the house, said "huh, the guy across the street mowed the front yard".

By the letter of the law it probably is, but if they hadn't expressly been told not to it won't go anywhere.