But here's the thing. They were legal. The ATF conveyed this fact. Law backed it up.
Then Vegas happened. Pressure was put on authorities to ban bump stocks. Congresspeople did their dances. Trump demanded that they be banned and the NRA seemed to be OK with that. However, nothing actually happened in the legislature. The executive simply told the ATF to make them illegal and they changed their interpretation of the NFA definition of "machinegun".
The Department of Justice is amending the regulations of the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) to clarify that bump-stock-type devices—meaning “bump fire” stocks, slide-fire devices, and devices with certain similar characteristics—are “machineguns” as defined by the National Firearms Act of 1934 and the Gun Control Act of 1968 because such devices allow a shooter of a semiautomatic firearm to initiate a continuous firing cycle with a single pull of the trigger. Specifically, these devices convert an otherwise semiautomatic firearm into a machinegun by functioning as a self-acting or self-regulating mechanism that harnesses the recoil energy of the semiautomatic firearm in a manner that allows the trigger to reset and continue firing without additional physical manipulation of the trigger by the shooter.
And with the clicking of a few keys, and some tenuous wordplay, they were made illegal. They are pieces of plastic with none of the parts of a machine guns as defined in the NFA. Yet they are now "machine guns". Not just when attached to a semi-automatic rifle. Not just when being used to bump fire. But just sitting there in their package on the shelf.
This is interesting. If Obama had done this, the right would be losing it's shit. If this was a different product, say IUDs and Trump told the FDA to classify the devices as prohibited narcotics, the left would be losing their shit.
But because it's the shitty bump stock no one cares. For me it's a frightening precedent (and president). An enforcement agency can stretch the bounds of all logic and proclaim anything to be anything else in order to make it illegal.
Personally I think it was a trial balloon and we can expect more use of this tactic. At least until even the facade of following laws goes out the window.