I would think and I'm only guessing that for a regular rifle even if you could make it work it would be highly illegal since you could easily slide it off and make the gun shorter than overall minimum legal length for a rifle. With the collapsible stock, the buffer tube still keep the rifle at legal length even without the buttstock on.
If I am mistaken on this, please correct me.
Another thing about that stock, if you push it forward it starts getting in the way of the bolt release amongst other things. Doesn't seem very practical to me.