> Mailboxes are typically outside of the fenced area where the dog is. Stay off my property and you won't have issues, is the point.

They typically are, yet mail delivery personnel are attacked by dogs constantly.

https://www.prnewswire.com/news-releases/us-postal-service-r...

That was just one example of many plausible scenarios. There are dozens of scenarios where dogs and people may be on different sides of said fence. The issue is an indiscriminate danger to people. It's the same reason you can't booby trap your own property.