My take on this (I have no official say of any kind, this is just my take on the situation. Agree or disagree with it if you like, it won't offend me. :) ):

- All WordPress themes must be GPL-compatible. Period. Any theme that is not is in violation of the WordPress license. There's really no room for argument on this, the license terms are quite clear, the precendents are straightforward, and any lawyer worth his salt will tell you the same. If you disagree with this basic premise, then you're simply wrong. Sorry.

- WP doesn't seem to be actively enforcing that license via lawsuits and such. Since they're the only ones who could possibly do such, people can get away with releasing said themes, but it's still a license violation.

- Their "enforcement" is (so far) limited to not including those themes on wp.org's theme directory. They're also actively rejecting themes that link to sites which have or otherwise support non-GPL themes.

- The depth to which they look for such "support" varies. Sometimes they won't notice a mere link to a non-GPL theme site, sometimes they will.

- Nevertheless, the bottom line here is simple: If you support non-GPL themes in pretty much any fashion, then don't expect to get your themes into the directory.

Note that this is all about licensing. It has nothing to do with selling themes. You can sell a GPL theme and that is perfectly legit and proper. So if you're a theme author and want to make money selling themes, but put others in the directory for free, then that's okay. As long as they're all GPL-compatible, then you're fine.

Additional: Yes, there are ways to get around the GPL. However, it is my opinion that in the specific case of a WordPress theme, this would be extremely difficult to do. I do know of one way, but I'm not telling. ;) Plugins have several possible ways to get around it if they really, really wanted to do so; themes don't.