This probably doesn't affect the argument one way or the other, but in Germany, if some company employs you, and you write some code as part of your work for that company, AFAIK, copyright for that code automatically belongs to the company.
There are even cases where copyright for code you write after leaving the company could be considered to belong to that company.
The basic argument is that the OpenZFS folks have not been assigning their copyrights to Oracle after the Sun buyout and are not subject to what you have encountered in Germany (which I find just wrong, sorry for your troubles). Perhaps LGPL would be a better license for an OS (might be an interesting discussion).
I have not had any troubles with this legal situation. When I am employed at a company that pays me to write code for them, I have no problem with the company owning the copyright.
And none of my former employers have ever tried to claim copyright on any code I have written once I stopped working there. Actually, I do not know of any specific case where this has happened.
There are even cases where copyright for code you write after leaving the company could be considered to belong to that company.