They aren't shooting at them. It used to be a popular thing for teenagers to whack mailboxes from moving cars using baseball bats or similar implements. It was a bigger problem ~20 years ago.
Of course now you're likely to break your hand/arm/bat if you try it. I've seen several methods used to prevent damage to mailboxes, including cages made from rebar, stacks of mobile home rims with a mailbox in the center, mailboxes made from drill pipe, the mailbox at my house is made from 1/4" thick steel plate and the post it is mounted to is a piece of I-beam. It's bonkers. I think you could hit it with a semi truck and the truck would lose. I didn't build it, it was there when I bought the house.