Why drupal.org doesn't host GPL-"compatible" code
While technically the GPL permits inclusion of code with GPL-"compatible" licenses in a GPL package as explained here, the Drupal policy is not to mix licenses. Drupal founder Dries explains this policy as follows:
Drupal.org's package management system automatically adds the GPL license to all packages. If we allow other licenses in CVS, it is going to get messy, and sooner or later we're going to run into licensing issues. Already, we get quite a few questions about Drupal's license. If we are going to add more licenses to the mix, it is going to be harder to audit, or provide answers to such questions. So, not allowing other licenses in CVS is a deliberate choice.
We've also decided against mirroring other projects in our CVS repositories--unless there are good reasons to do so. So when people need a non-GPL library, it is best to instruct them to download that library from that project's website.
So, in short:
- Only GPL'd code
- But no GPL code that is available from 3rd party sites as long as the proper version to use with your module is easily accessible. Easily accessible means that the release that works can be downloaded in a compressed file of some sort. If your users have to use svn/cvs to get the files then that is not considered easily accessible.