What do they need semi auto, high capacity, rifles for?
Hunting pigs, for one. They're a real menace in the southern USA. Bears, coyotes, wolves, and other wildlife for two. And defence from other people for three. A lot of people don't realise how big America is. In much of it you can't just ring 999 / 911 and expect a quick police response. The police may not be able to turn up for half an hour - or more - even if they respond immediately. And a hurricane disaster a few years ago in Louisiana showed, the police can be just as bad as looters when they do show up.
Edit: here's a video about shooting wild pigs:
look how large some of the herds are.