Doesn't surprise me. 99% of people out there have never heard of 922(r). They only know about the Assault Weapons Ban. 922(r) has to do with importation restrictions, and it prevents you from modifying an SKS to be in the same configuration as a rifle banned from importation. SKS's with detachable magazines are not importable.
Check out this SKS FAQ Page
for more info. It's packed with good information.
I'll bet that when you ask the guy about the detachable magazine, he gives you a blank stare or mumbles something about it being OK under the Assault Weapons Ban. Unfortunately, section 922(r) is not tied to the AWB and does not have a sunset date.