And he can actually be arrested or fined for doing that as he is intentionally trying to cause damage to other people's property. Just FYI.
Nah, in most states the closest you'd get to that is the charge of "malicious destruction of property" and IMO there isn't a judge or jury that would find malice in fortifying your own mailbox that is continuously being damaged.
Usually the issue isn't the plow blade physically hitting the mailbox or post--it's the force of the snow/slush being flung at the mailbox from the plow going too fast.
Two winters ago I was driving behind a town plow and mailboxes were going down like dominos as the plow passed. You know the driver saw it happening in the mirrors.
Then there are guys like this...