I understand your point about the receiver, but it doesn't work like that on some guns. That's how it works with the FAL, but this is because the ATF had determined that converting a semi automatic receiver to accept FA parts is "beyond the scope/ability" of all but the most competent gunsmiths. They don't care how many FA parts you have, as long as the receiver isn't prepared to accept them in any fashion (Still talking about the FAL here).
The AR15 and M16 share almost identical receivers. I think the only difference is a sear pin hole. Thus, the ATF considers it to be within the scope of the average person to convert it. Ergo, it's considered a machine gun if you have the FA parts immediately accessable. They don't even have to be installed for you to be charged!
My point is that you shouldn't assume that because the lack of a receiver is OK on some guns, like the FAL, that it the same with all types. Not so!
If this wasn't true, I'd have one of those kits, but I'm not touching it otherwise!