Gun Control: The Law Says what we Want it to Say edition

At Trump's directive Jeff Sessions and the ATF just put out a new interpretation of the NFA that bans machine guns.
This is, of course, a hilariously bad idea, even if you think that gun control in general is a good idea.
Don't understand why? Well:

1. This action relies upon deliberately bad logic to re-define bump stocks as machine guns. Basically, they're lying about bump-stocks being covered.
2. Bump stocks have been legal for over a decade. There have been MULTIPLE ATF letters that said they were legal.
3. The ATF is taking the position that they were always illegal, they just didn't realize it. Therefore the takings clause doesn't apply.
   

So, the government decided that an item met the definition of another, illegal item, despite the language of the law not supporting that interpretation, and retroactively banned it after over a decade of it being legal, while denying compensation, because it was always illegal. Now imagine if they did this with anything else. If this is allowed to stand as precedent, you can be damn sure that they'll do it again.