We don’t have to speculate, we have a precedent to look at. Back in 2010, Chris Pearson (Thesis theme) and Matt Mullenweg had a debate on Mixergy about themes that are not GPL licensed. If anyone is interested in watching the video you can see it here.
Pearson claimed that he can license it whatever he wants. Mullenweg claimed that the provision in GPL requires plugins and themes to be GPL licensed.
Fast forward a few years, Thesis theme is split licensed as I mentioned above. PHP is GPL because it relies on WordPress. CSS, JS, and images are proprietary.
I’m OK with either option, require all themes be GPL licensed or allow split license. But, in no way we should allow themes that are not GPL licensed to be part of the directory.
They can do whatever they want on their own website, but being part of the directory probably should require full GPL license.
And if we were to allow split licensing, proprietary code should be open source so theme reviewers can audit it.
Edit: Chris Pearson is apparently behind DIYThemes, whose Twitter account called me a moron for suggesting/mentioning ClassicPress recently 
He’s got a lot to say about WordPress theme system:
The current WordPress theme “system” was released in 2005 and isn’t really a system at all—it’s a file naming convention run by a big
if-elsestructure. It provides essentially zero functional leverage for the things Theme creators need to do.