The Left never starts from the premise that individual Americans have a constitutional RIGHT to keep and bear arms. They don't want to read the Supreme Court's majority opinion in "Heller" which spells that out and removes the notion that guns are only for a "well-regulated militia." They don't care that in Roe v Wade the high court went hunting to "find a right to privacy in the penumbra" of The Constitution. They got what they want. In Heller, they didn't so they pretend the ruling doesn't exist and begin from there.
Since Scalia left room for a possible ban on SOME forms of guns, that's all the Left saw.