Because they were a self-employed nonresident alien living in the United States not in any exemption period.   So as far as I know they must pay SE tax because they were subject to an international social security agreement which determines that that they're covered under the U.S. social security system. 

I appreciate the comments & questions but I have a note to post this on the ProSeries Tax Idea Exchange instead of here (whenever the 1040NR is released) because I really just want them to correct the calculation on the Qualified Business Income Component wkst table and line 1 on the 1040NR package in case if it ever comes up again.   

And they didn't fix it when I called them last year about it.   I don't like when I have to eFile returns bypassing error conversions and also prefer that my clients not get letters from the IRS.