Time. To be short and simple, when they passed that dumb law they knew they could not ask everyone to turn in their old big mags. So they only passed a law against new mags over 10. Any magazine made before the ban date can be owned, sold, used, imported, whatever (though a few states have their own more restrictive laws) but no magazines made after ban date can be larger then 10 rounds (for civilians anyway). Unfortunatly the CZ Standard IPSC was not made before the ban and neither were its mags. Thus all its 16rd mags, made after the ban, are a no-no for us here, while the SVI/STI old mags are just fine.
What happened was that right before the ban, everyone made millions of mag bodies and they are still selling them at a sane price. And it gets even funnier, because you can buy new bodies to replace "worn out" ones.
Its all a mess.
Vlad