ATF had a shit ruling that says any gun in a holster thats designed to be fired from the holster without removing it first, is an NFA device known as an AOW, or "any other weapon". Basically it hinges off of another b******* definition where basically things that don't look like guns that can fire bullets are AOWs.
That's why when you see gimmick guns like a lifecard or those pen guns they always fold into a shape that looks like a handgun of some sort.
Now having said that there are other pen guns like a stinger or whatever but those are actually NFA devices.