I think I remember reading something (no idea if accurate) that said that the legalese ATF used made it not tax free, but more like tax deferred, and that they could come after people for the 200 ducats. Or the IRS could, because the ATF isn't allowed to waive taxes (as per what I read).
Though I don't think they would. They used that as an incentive, and if they were to turn around and suddenly say they want their money back (hey, this sounds analogous to their about-face on braces), then good luck the next time they tell people, "Hey, trust us".