That is due to licensing reasons, yes. It makes maintaining the codebase even more complicated because when the kernel module API changes (which it very frequently does) you cannot just adapt it to your needs, you have to work around all the new changes that are there in the new version.
You have things backward. Licensing has nothing to do with it. Changes to the kernel are unnecessary. Maintaining the code base is also simplified by supporting the various kernel versions the way that they are currently supported.
