TexasCHLforum.com

Bump stocks or binary devices does not turn a semi-automatic to a machine gun. As machine gun (fully automatic weapon) one-press on the trigger multiple-bullets fired)
So, If Obama administration said it is legal, nothing changed to make it illegal.

2 things they can do:
1) require further checks and may require a registration and any transfer is via and FFL
2) Conclude it is not illegal to own and the only way to ban or regulate the use is via a new Federal Law

2) is more likely as such devices are accessories and it would arbitrary to regulate said devices rather than say flashlights or optics