One thing I once heard, which is so true: There is a huge difference between handling a shooter and handling a real gun (compulsory military service anyone?) AND there shouldn't have been access to firearms in the first place, which is the most crucial thing...
Thing is: There IS a conditioning going on while playing games (as is for anything else), there is no denying that... But (and that's a big "but") what the politicians and the media conveniently ignore is, conditioning doesn't mean brainwashing... So a normal person is able to see the difference between the game and the reality and can draw a line between the two, as they would with a film, a book and so on...
So while it surely is convenient to have a universal scapegoat for everything that is wrong with todays society, it's never that easy... And while I'm all for enforcing those ratings, banning inconvenient things is no solution... It just makes things more desireable than they already are... Plus, as I said earlier, I don't think the general public has realised how big gaming has grown and that they might shoot in their own foot...