It is illegal because in 1934 they passed some laws pertaining to SBRs and Sawed off shotguns etc. Part of that law says that you can take a pistol and make it into a rifle but you cannot make a rifle into a pistol. It was all about the ease of concealment back in the days of mafia thugs wearing overcoats with their Thompson slung under it. That was decades before the AR15 came along and they have never bothered to update those laws with the AR in mind.Just another version of "if you aren't doing anything wrong"
If the only applicability is in conjunction with a more serious offence, then why is it illegal?
Well I know the answer.