The nice thing about the US is we don't "offer to form citizen militias" to stop crap like this--we just form them. The Rodney King riots in LA stopped when the rioters ran up against just such a barrier of armed citizens. Thank God the horrible AWB expired.
I don't recall this being the case at all (though there were specific instances of business owners hunkering down with their families and their guns to protect their property). I believe the LA riots stopped when the U.S. postal service announced that it wasn't going to deliver mail -- including all kinds of government checks -- to South Central L.A.