mrtd: "Open source is copyright free in the sense that anyone can freely make copies of it and amend it for their own purposes."
No no no no no.
The code is copyright the author, this forbids you from making unauthorised copies. You are authorised to make copies if you adhere to the terms of the licence.
If you don't adhere to the terms of the licence, you are not authorised to make copies of the code. Therefore you are distributing copies without the permission of the author. Therefore you are violating the author's copyright.
Any lawsuit involving the non-compliance with an open source licence would be a copyright lawsuit.
Copyright is the *backbone* of open source and free software; not its enemy.