The concrete mailbox ban isn't there due to a booby trap ban.
There are a lot of regulations regarding what can be near roads as their presence can provide a potential hazard to traffic. This includes a mailbox. Mailboxes are designed to be within the clear zone, the area that needs to be kept clear of obstacles, in order to allow mailmen to deliver mail to them. So, in order for them to be within the clear zone, they have to be designed to break away when hit by a car.
This has nothing to do with the intent of the person hitting the mailbox.