No, they certainly take original work by independent designers that is GPL licensed and doesn't have any spam links or anything.
I don't know what the specific issue with Artisteer would be. The themes might not be Open Source, or they may not support a feature like sticky posts. Or it could simply be that the themes are not sufficiently original enough to be included.
As far as I know all of the themes on WordPress.org are created not with a generator, but "by hand" with a text editor or with a WYSIWYG editor like Adobe Dreamweaver.
Here is the criteria for getting your theme hosted on WordPress.org.
But don't let that discourage you! If you have a great looking theme that is simply lacking threaded comments or some other feature share it from your own website. The cream will rise to the top :)