It's easy and not really up to interpretation. That's why we have licenses, to eliminate the interpretation part.
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The WordPress.ORG software is free to use and licensed under the GPL. The GPL has a restriction: if you use GPL'ed code and distribute it then the source code must be available.
Example: if I took WordPress as in the http://wordpress.org/latest.zip and I modified it and just used it, that's allowed. That's also a trivial case which I'm just including to be complete.
Now I modify the WordPress.ORG softwate and distribute it. My changes are all there and anyone can read them. I'm still within the GPL since I anyone can read what I've modified just by looking at the PHP files (source code).
Now I take the whole WordPress.ORG PHP files and run all my changes through a tool to obfuscate and encrypt the whole code and I distribute that unreadable code. That would be bad.
When I distributed that obfuscated code without making the source available, I have just violated the GPL license that WordPress is distributed on.
Now about those premium themes and plugins.
Let's say I wrote a kick-ass plugin and theme. That's not GPL'ed code. It could be, but it does not have to be. Just using WordPress functions does not make my code GPL. Neither does having a functional dependency on WordPress code make my code GPL'ed.
It would be nice if I did GPL it but it does not have to be since I am not distributing WordPress.ORG software. Using function calls from the codex is not distributing the GPL'ed software.
I wrote it all myself and I can dictate any thing I want with my code. Don't like my terms? Then don't use my code. But feel free to do what you want with the code that came from WordPress.ORG within the terms of the GPL.
This also applies to themes with evil footer crap. They can and will do what they like with the code they have written. The solution is to stop using code that you don't like.
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Hope that clears it up. Let's not mix up what software developers should do with what they must do.