I'm kind of conflicted on this. First I think the whole SBR/SBS portions of the NFA are ignorant to start with...well, the whole NFA is ignorant, but anyway.
I could see the writing on the wall with this when it first came out and the ATF Tech Branch approved it. People were going to use it as an improvised stock, thus negating the original intended use for the device and the reason the designer made it that way. Here's the actual inventor and why he did it:
New Stabilizing Brace for AR15 Pistol "SB15"
Which is a great reason to design such an item. But as soon as they hit the market, you knew the ATF would reverse course on the decision since people continued to push the limits of what the design was intended to do as opposed to what they were using it for. So a designer, Black Ace Tactical, specifically designs an SBS using the Sig Brace as a stock. Which in turn gets the ATF looking at it yet again. And being that the shotgun in question uses said brace as a stock (against the original design) the ATF reverses it's original ruling even knowing the brace was likely to be used that way from the start and acknowledged that fact.
So I'm torn as the ATF was doing the proper thing, even though I don't support the NFA. But I also blame the company that used the brace as a stock as pushing the envelope to have to put them into that position. Just like the idiots that couldn't leave well enough alone on the 5.45 Russian and
had to make a pistol, thus causing the ATF to classify all 7N6 surplus ammo as armor piercing and deny further imports. So the cheap spam cans of Russian surplus dried up virtually overnight. Gun owners are our own worst enemies from time to time and I can't blame the ATF for being forced into the position they are in because a manufacturer didn't know when to say enough.