Well, for a lot of people, a braced pistol IS a practical firearm for certain situations (home defense and truck/trunk gun come to mind). A 9mm, braced AR or AK could make an excellent home defense firearm and whether the brace is used correctly or not, it would make an effective tool for people who can't wield a full-sized rifle, or large pistol effectively otherwise. I agree with many that a <10inch 5.56 AR is nothing more than a noisy distraction, but there are a lot of braced pistol options out there that really are more effective firearms with practical uses. The approach taken by the ATF is simply one of jamming a stick into a hornet nest for the sake of doing it. Yes, some manufacturers are obviously by-passing the unconstitutional NFA, so identify those products and have them removed from the market. Don't put out a non-binding letter of intention that says, "Register your firearm pre-emptively, or submit each individual weapon you own with a brace to ATF for determination, or take the brace off the weapon." ATF can look at the brace options on the market and determine what they will and will not allow... as others have said, they are just trolling at this point.
There is some speculation that certain high rollers at ATF are also trying to win favor with the likely incoming administration... If true, that's even more sick than being arbitrary. Placing this kind of burden and possible state sanction on otherwise law abiding people, for the sake of personal career advancement, is the very thing that makes people distrust their own government and its motives. They should try applying the law, as it is written, in a manner that is consistent with the best interests of the effected population. There has got to be a crime that produced a real-world victim that they could be investigating instead.