I think it's just that Windows Containers are a lot unlike Linux Docker containers; they're not useless, but they're very different. I don't think Darwin containers have to be so different from Linux containers, not the least of which because Darwin is UNIX. It would be a lot more like if you ported Docker to run natively on BSD than Windows, since it basically is that.

An easy solution to the licensing issue is just having an open source userland that you can use, which Darwin sorta does. That said, since XNU syscalls are not stable, there is some awkwardness here. Either they would need to be made stable somehow (e.g. by providing something like personalities, where different processes get different syscall behavior) or the runtime would need to drop libc/base libraries in from the sky and containers would just need to assume they're available at a specific location. Either thing seems quite viable if Apple wanted to do it, IMO.