As a nation, we've decided that we don't want to restrict the rights of hundreds of millions of law abiding citizens merely because a few people misuse those rights. This is why politicians have generally decided not to push for more gun control - it's an election losing issue.
Incidentally, I'm confused by the point of that tweet. Here are some other .223 rifles:
http://s845.beta.photobucket.com/user/OldColdWarrior2009/med...
http://media.liveauctiongroup.net/i/9380/10373165_1.jpg?v=8C...
http://molot.biz/product-e/vepr223-super.jpg
For most calibers, you can find guns with many different form factors that accept bullets of that caliber. So what?
