Oh....we had that discussion with the CFO inspector when we were getting our firearm dealers license. There is exactly ONE way to pull that off. You MUST have a contract with a government agency that is allowed to buy unpinned magazines. Even then, ordering in a bunch in that aren't done, and pinning half of them to be sold to the public is not allowed. That being said, I know of at least one retailer who's doing it quietly. No, it's not us. No it's not Marstar (that I know of) and I'm not going to elaborate because that's just being a rat. I just know it's being done.
The official answer is NO NO NO, they aren't allowed to do that.