Regardless of whether the original inventor had other intentions, two things are blatantly obvious:
1) Sig never would have brought it to market if they thought the only market was one handed amputees who want to shoot AR pistols. They can say what they want, put up pretty pictures of whatever they want, but their intended market is faux-SBRs.
2) No one who buys one does it for any other reason than to avoid SBR hoopla. They bought it as a stock, and use it as a stock.
Now I'm no fan of the NFA, as I'm sure few here are, and couldn't care less how anyone uses their "arm brace", but how this one slipped past Obama's ATF I'll never know, and have to believe the day is coming when they will "fix their mistake". Just like with past mistake fixes, the buyer of the now illegal item is out the cash. At least this is a relatively cheap item.