>

The idea is that they’re easier to conceal. Same reason you can buy a rifle in 15 min (in some states) but a pistol still requires a longer background check. Useless law either way IMO
FYI people in CT, IL, and Cali are extra fucked due to this law. They're not allowed to own SBRs as they can't own NFA items. Guess what happened to the "Others" they were legally in possession of? They're now NFA items, and since they can't own those items in those states...
I'll also note the ATF has said these braces in the past were legal when they were written letters asking about them. 
Personally I completely disagree with this ruling. Many of us firearm owners in CT, Cali, and IL are in the process of having tragic boating accidents (tm) as we speak. I imagine it'll be struck down eventually. I really don't like this "yeah those are legal, you can own them" "NO WAIT THEY'RE NOT! FELONY TO OWN!" bullshit they're doing. It's not even legislation that was passed, BATFE is just deciding on a whim.
EDIT: 
Straight from the document. 

In a narrow set of circumstances in which the individual cannot remove the
“brace” device and maintain the pistol, or make other modifications to the firearm, then
the firearm may be prohibited under State law and must be either destroyed or disposed
of in compliance with State law. The Department notes that commenters did not appear
to submit information indicating the frequency with which such circumstances would
arise, and ATF’s experience indicates that these circumstances would be extremely
limited. The Department thus believes that the important public safety benefits of this
rule, as discussed more fully in the accompanying Regulatory Impact Analysis (“RIA”),
outweigh any interest in retaining the firearm for the few individuals who might find
themselves in such circumstances.

So "Public safety benefits" outweigh me being able to keep my legally owned in compliance at the time firearm? Why comply? >

“No one is coming for your guns lolz” is what I’ve been hearing for years. Well since my state bans SBRs, I have a choice to either get rid of my AR pistol or become a felon. Thanks >

As a European why exactly are short barrel rifels such a threat to society? Compared to either pistols or long barrel rifels.