You’re missing the poimt. Trump directed DOJ and ATF to ban bumpfire stocks. When ATF implements a regulation they first issue a Notice of Proposed Rulemaking, which states what the new regulation will be and when it will become effective. The Notice also includes a comment period, when the serfs are allowed to talk back to their betters. In rare cases ATF has actually changed the proposed rule due to feedback. The issue here is that ATF has been directed to change the legislated definition of a machine gun through regulatory action. Constitutionally, this MUST be done by congress.
Federal Register - Bump-Stock-Type Devices. There is a lot of information in this link.
923-Page Opposition to ATF “Bump-Stock” Ban Filed.
I suspect the case will go all the way to the Supreme Court. Since the legislative branch of government has ceased to work, I suspect they will remain silent and not do the job they are supposed to do, which is make the laws. Which is why the courts have garnered more power than they should have.
But President Trump is certainly not doing it on his own. He is working within the Constitution. President Trump may have directed the ATF to re-examine their ruling, but that also is legal. They did and decided that bump stocks do meet the one trigger definition. Lawsuit was filed against that ATF ruling, and is now proceeding through court system.
We may not agree with the final outcome, NRA has no problem with banning bump stocks, but at least it is an orderly process. It does, however, expose the uselessness Congress has become, they no longer do their job at all. Why? Because they are afraid they will not get re-elected. Makes for a good case of term limits.