I think they have said somewhere that Headscale is actually revenue positive for them.

I have the same memory. But they may not feel that way forever. Many a company started by attracting developers with a generous free tier or open-source offering, then started to clamp down once the going got tough.

Heck, it happened to one of Tailscale's competitors, ZeroTier, which used to release their client software under GPLv3 but eventually switched to BSL.