When I was 8 years old I made my first improvised weapon. (I was/ am a little strange...) I took a sliver of soup can and stuck it on a crude handle with flooring nails. It had about a 2" spike that was pretty damn sharp.

Here's the actual letter from DOJ / ATF.
 
http://www.kennesawcannon.com/images1/batf.pdf

Seems to be saying that simply owning this device has nothing to do with firearms laws.  Though, when you put it into the flare gun, you have entered such territory.

They seem to be providing this letter with purchase of the item.

It seems to me that you could buy these things all day long with no firearms paperwork or legal concerns.  Only when you "create" a firearm by combining the sleeve and flare gun, at which time you are expected to comply / register / etc.

It seems that it would be a good doomsday / confiscation scenario to buy several of these, flare guns to match, and ammo to match.  Then keep all 3 in different containers.

If you get to the point in social disintegration where you need to put them all together, it's probably long past worrying about BATF and generic legal issues.