Let's say that I'm employing you to write a trading platform. You research the field, find some OSS that can serve as a base to work from and modify it to my specs. I own the modified code and can report you to the DA if you take it and try to sell it somewhere else.
Now if instead, you just take the original OSS you found, and the knowledge you gained working from me and go somewhere else to build a trading platform based on that original source, I have no case against you (assuming you aren't using my trade secrets) since you haven't taken anything I own.
> I own the modified code and can report you to the DA if you take it and try to sell it somewhere else.
How does this work with the original copyrighted code? If I take 10k lines of an open source project, tack on an echo or a comment or some nonsense, do I suddenly get to claim ownership of the rest of the code? It would make sense that you could own the delta, but you should only own your delta, not the original code.
The point of GPL is that you own the entirety of the source with your changes. Remove your changes and you're back to the original source which everyone "owns."
"Owns" in quotes because that's not ownership, it's a license to use, modify and distribute the code within certain limitations. I've been dealing with IP lawyers recently over exactly this issue and they're being sticklers over the difference.
If a piece of code has parts created/owned by different entities, in order to copy them you need permission from everyone of them. You copy that resulting 10k line program but don't have permission for my delta that's a single comment? You're violating copyright.
Let's say that I'm employing you to write a trading platform. You research the field, find some OSS that can serve as a base to work from and modify it to my specs. I own the modified code and can report you to the DA if you take it and try to sell it somewhere else.
Now if instead, you just take the original OSS you found, and the knowledge you gained working from me and go somewhere else to build a trading platform based on that original source, I have no case against you (assuming you aren't using my trade secrets) since you haven't taken anything I own.
Does that help?