I see a lot of mailboxes on heavy steel posts set in concrete, so that the plows don't wipe them out. I don't believe they are legal for the reasons you mention but nobody says anything.There has been a few bad accidents over the years in Illinois where someone hits one of these large mailboxes and launches it in to someone or some other bad result. I think the state dot enforces just a regular mailbox on a post but you still see some different ones on backroads.