Grief does not lead to rational decisions. That's stated often by even Lefty Psychologists. The parents need to lash out, to "do something", to make their children's meaningless deaths meaningful. They do this by attacking guns.
Had their children died in a bus accident would they be calling for the banning of buses and cars? Some might. However most would strike back by litigation against the school and anyone else they could.
How little you understand human nature.