Should someone decided that they'd start working through the code, removing duplicate code and clean up headers, functions and abstraction, they work would either be rejected, or undone with the next AMD code dump?
Sure, you can fork the code if you really feel that strongly about it. My main "issue" is that it basically removes one of the big benefits of open source, that we can collaborate and do better as a collective. If it's just a big code dump that other kernel developers can't really touch it's more "source code is available" than actual open source.
When you send someone a PR, you are demanding that they do work for you to review and merge. Open source licensing does not mandate that they do this. Heck, most open source licenses even disclaim warranty to avoid obligating the authors of even doing work that the law would otherwise require them to do. Now yes, some people will help you with problems. This is because they're nice, not because it's open source.
Because it is open source, but not open contribution.
Open source vs. open contribution are orthogonal concepts: the former is about licensing, the latter about the organization of the development process.