"Personally, I am surprised that the ATF didn't just outlaw their possession without a tax stamp and be done with it."
^^ We have a Winner.
This is exactly what the BATFE *should* do, in my humble opinion.
Wrong. The brace itself could never, should never, will never be an NFA-regulated item of any sort. Sorry, it's at worst a buttstock, same as any other. The ATF should have just sent out a PR bomb over a few months saying that attaching the brace to a pistol will be grounds for it to be confiscated (and the owner possibly prosecuted) as a short barrel rifle. A few months of letting word get out, and there would be as few people putting braces on pistols, same as there are people putting Magpul stocks on pistols illegally.
In fact, this dubious latest decision may be the ground work for exactly that;
1-Accidentally rule the brace is not a stock out of incompetence (Hanlon's Razor)
1.5-Individual agents issue rulings based on Bureau precedent in keeping with #1, stating that use of the brace on the shoulder cannot be regulated
2-Ignore the oversight until it becomes a glaring example of said incompetence (more Hanlon's Razor)
3-Come up with a way to rectify the oversight in a way that reflects as little blame on the Bureau as possible (politics)
4-Use the tried-and-true tactic of assigning rules to "intent," which the ATF then has pretty much full reign to define/prosecute until such time a court reverses them ("intent" of using the brace as a stock when attaching it becomes prosecutable)
5-The users thus "primed" for additional ATF statements on the SIG brace, muddy the waters for potential/current users further with a grandiose and unenforceable proclamation before a large media event feature them.
6-Wait for sales, use, and interest in the brace to collapse, and the companies making them to voluntarily cease production due to lack of profits (and hope they can hold off inevitable lawsuits long enough for that to happen)
7-Once the manufacturers of the braces cease production, there is no organized party to file suit in court, and the ATF can make their final announcement with little opposition or acrimony;
8-Declare the brace a stock, and contraband to attach to a firearm or other non-long-gun. Publicize a few prosecutions/confiscations to show they're serious, and the few remaining holdouts will fall in line, utterly unable to oppose the move without risking life, liberty, and property.
Skipping steps 3 through 7 would be clearer for everyone trying to obey the law, and better in the long run for all involved, but the extra steps cushion the impact of the Bureau admitting it made a terrible mistake (and implicitly admitting that even they can't make sense of the NFA when it comes to SBRs). The muddying of legal statuses also makes it harder for the affected parties (us and SIG) to claim standing in court, compared to if the ATF simply ordered SIG to stop manufacturing the braces, or an assortment of companies to completely re-jigger their marketing strategy at the stroke of a pen. The 'scenic route' described above lets the manufacturers continue production "unimpeded" so they are less likely to convince a judge of standing/damages.
TCB
) and said the shoelace was not considered "intent" for the purposes of prosecution unless attached to the firearm the bad (read: probably fun) way. Simply too many common legal uses for a shoelace (Boresnake, anyone?) to deem its mere proximity to a firearm as "intent" in the courtroom (not to say that it couldn't figure in among other evidence, but the lace itself was no longer sufficient proof unless affixed to the gun). There were some folks who registered their shoelaces, though, but mostly to say they could 
