It's a matter of principle. 

Via Townhall: 

No person shall be . . . deprived of . . . property, without due process of law.

-- U.S. Constitution, Amend. V

No . . . ex post facto Law shall be passed.

-- U.S. Constitution, Art. I, Sec. 9

Stroke of the pen, law of the land. Kinda cool.

-- Paul Begala, Advisor to Former President Bill Clinton, July 5, 1998

Two bulwarks of individual liberty – that the government cannot seize a person’s property without due process of law, and that it cannot prosecute an individual for an action that was lawful when the person performed the act – are threatened by a single regulation now pending before the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), a component of the United States Department of Justice.

The regulation is designed to prohibit people from possessing so-called “bump stocks,” as apparently had been used by mass murderer Stephen Paddock last October 1^st in Las Vegas. While the intent of the proposed regulation may be considered laudable by many, the procedural precedent it would set should cause serious concern for anyone who supports limited and accountable government.

Read more here.