Fans of Alternity

See, given what Rosier said to Bill a while back, it just makes me wonder what the long game is in pretending that Muggleborns steal magic and that keeping them in the camps somehow prevents that. I mean, do some of the less sophisticated Death Eaters--Corax rather comes to mind--actually believe it? Does Voldemort? Is this a rehash of when Bellatrix was sure she had killed Sirius? Mysteries probably doesn't honestly believe that birth rates would be tied to the new policies, since they're sort of working on stealing magic back.
Or are they so eager to believe that it is true that they'll accept the lack of baby Muggleborns as proof, which is how it looks right now? Because that could certainly work to the Order's favor.