There are lots of stupid laws on the books in many states...
PA just seems to have a few more than most states...
Not only is it illegal to hunt big game in PA with a semi auto rifle ,pistol, or shotgun but it is illegal to hunt ANY game with a semi auto rifle or pistol...That means you can't shoot a woodchuck or a squirrel with your Ruger 10-22, NOR can you shoot a coon out of a tree with your vintage Colt Woodsman..
It is, however, perfectly legal to shoot game birds, small game or varmints with a semi auto shotgun and small shot, as long as it is plugged to hold only 3 shots..