Very simply - neither the ATF nor the DOJ have the authority to reinterpret the law. The law covering Machine Guns is VERY explicit and can only be changed by legislation - congress - not by an unelected bureaucrat and a pen. As the existing law reads defining what a Machine Gun is, these do not fall under such.
Another issue is there is no grandfather clause - it simply demands that all bumpstock owners surrender or destroy them. Devices that for over a decade the ATF THEMSELVES wrote on MULTIPLE OCCASIONS to both owners and manufacturers that such were legal and furthermore testified they did not have the authority to change that. No compensation is being offered. This is just the government coming in and saying - give us your property or destroy it. This is in violation of the 5th Amendment - private property [shall not] be taken for public use, without just compensation."
This also violates the 14th Amendment, namely Due Process under the law. Bump Stock owners have received no due process. Just a demand for compliance and obedience. That is not acceptable.
This is going to be a short-lived moral victory for gun control advocates.
The other frequently overlooked issue that a 'bumpstock' device can be emulated using a couple of rubber bands or a belt or created with things lying around the house.
I understand the outrage after the Vegas shooting but this is a useless ban sought by people that really don't understand what they're talking about.
At some point we have to get past emotion and start evaluating facts.