I think it's considered to piggyback on the 16th amendment -- the IRS collects it just like the IT. (I know, I know, don't tell me that the IT is unconstitutional).
Well, now that you mention it, I DO have my concerns about that, as well as the 14th & 17th Amendments, too...& the federal government doesn't wanna discuss the questions regarding the 16th Amendment, either. WHY WOULD THAT BE??? What have they got to hide?