I expect they have reasons for thinking they've succeeded in preventing magical births in the muggle camps.

Only, they must be using charms or potions or something to try to prevent that. Experimenting with various things, I'd think. And if they are, then they'd be likely to think that the lack of muggleborn children is a sign that whatever they've been doing works.

I don't know any of that for sure, mind you, but it's my guess.