Because it is not an ex post facto law. Cocaine used to be used in the manufacture of Coca Cola. Possession of cocaine was at some point in history prohibited by law. It did not make past possession of cocaine illegal, only possession following the enactment of the law. It did not criminalize or punish any activity prior to enactment of the law which is what the prohibition of ex post facto laws prevents.
There are those who want to pass laws prohibiting all kinds of firearms and accessories for firearms. If those laws are passed, it will not make the prior possession a crime but make your continued possession a crime.
That said, it bothers me as much as anyone else. As far as 2A goes, I would probably be considered libertarian. If someone wants fully automatic weapons, have at it. Tanks and bombers, good for you if you can afford it. Live and let live.
You can find more on this issue in the following thread Bill of Attainder - #11 by MikeBKY