My thoughts on the subject.
First, I've always been surprised that the ATF ever allowed any kind of brace for AR pistols. I've seen lots of these at the range, and every single one of them has always been fired from the shoulder 100% of the time. I know several individuals who have either avoided having a brace, or registered their weapon as an SBR to begin with because they always anticipated that this would happen.
I get the idea that someone was trying to help people more accurately control an AR Pistol with one hand, but even the original braces were clearly configured to also be fired from the shoulder. We can argue against it by asking how it's possible that they could change their minds after allowing them to be sold for nearly a decade, but we will loose that argument. The ATF and DOJ should have said NO when they reviewed them the first time around. The fact that they did not do so does not mean that they can't do so now.
The root of this issue is that an organization such as the ATF, or even the DOJ is left trying to classify what falls under the NFA and what doesn't. That is not acceptable, and I would say that it's not even legal. There is a reasonable expectation that a LAW be clear enough and specific enough to avoid this kind of ambiguity. With every law there will be SOME level of ambiguity, but this is nonsense. This is not something that was unforeseeable in 1934. The fact is that the NFA and the GCA were both intentionally written with the purpose of allowing the government to "interpret" the laws as they see fit in order to accomplish their agenda. That agenda being restricting the American Citizen's access to "weapons of war" that could be used against government officials.
This brings me to my 2nd point. Regardless of whether these laws were written with ambiguous language on purpose or by ignorance is irrelevant. The NFA and the GCA are both 100% unconstitutional. "the right of the people to keep and bear arms, shall not be infringed." is the single clearest piece of language penned into the entire Bill of Rights. Even the 2A's purpose is clearly spelled out. We the people have a God given RIGHT to "weapon's of war". This right, and the willingness to use it is what defines a person as being free.
If our court system, specifically the SCOTUS, refuses to fulfill their sworn oath of upholding the constitution, and protecting our God given rights, then we've either reached the end of our Government or the end of our Freedom. The fact that millions of braces have been sold and installed in a formerly legal fashion on AR pistols will most certainly give us an opportunity to test whether or not the SCOTUS will do its job or not.
Given it's recent refusal to hear Texas's lawsuit over the election, where some states unconstitutionally changed their election procedures, I'd be willing to bet on what their decision would be.
That's better.
Paragraphs, man.