Thats a remnant of the old "Saturday Night Special" gun laws, they used funny criteria to prevent those types of compact, low-end manufactured pistols from being marketed. The reason that dealers are restricted to sell is that an "ex post facto" after the fact law cannot(should not) happen.
I.E. any of the guns that already existed in the state in the hands of private owners were legal at the time of sale and remain that way. In-state private sales have very little restrictions on them, and no reporting requirements, so it was not part of the Bill.. they could only monitor/enforce it amongst dealers.
Just like the assault weapons ban in the 80's, they had to restrict the manufacture of the firearms(in this case the sales) after a specific date, they could not make all previously existing variants of the guns illegal.
Once the AWB law was enacted, there was still a delay before it was fully implemented.
So all of the magazine manufacturers ramped up production prior to the enactment date and made enough high-capacity magazines leading up to the ban to offset the pain. The other reason that SKs's were so popular at the time, EVERYTHING about them was pre-enactment, although they eventually made the importers remove the bayonets.. Prices climbed a bit and you still see people refer to ARs as "Pre-Ban" because anything preban was legally allowed to retain all of its "Evil Features" Collapsible stocks, flash hiders, pistol grips, high cap mags, etc, etc