If they're not doing a 'clean room' rewrite, I'd agree. Otherwise? That's their choice, and it's one I'm glad to see. This could be quite useful for OSes that aren't GPL'd, like the BSDs, or Redox.

What is it with rewrite-the-world-in-Rust people being stridently anti-copyleft?

I don't understand it either. It's as though they want to ignore the history of free software, which simply wouldn't have happened without copyleft.

Tautologically, it wouldn't have happened as it did, but free software long predates the FSF.

Sure, but the FSF and the GNU project were reactions to proprietary software becoming more common. Free software wasn't recognised as such until Stallman saw the issue and decided to act. Saying that "free software existed before the FSF" is ignoring the fact that free software would've died almost entirely without the FSF and GNU.