When I read code under GPL source and write something like that under a different license, I'm legally liable for copyright infringement. Of course the original owners need to prove it first, but still there's problems from that.

Neat. But if you create your own version based off what you read that's fine. You can't copy it, but you can learn from it.

I can read the Linux source code and use it to create my own compatible kernel.

Some open source projects outright disallow you from contribution if you tell them you're working on a closed source competitor.

So? They can refuse submissions to their code but they couldn't stop you from using what you see to create your own product.