[tsimionescu]

Think about it like this. If I redistribute a BSD licensed piece of code released by the OpenBSD project, I have to include a note saying that "this product includes code from the OpenBSD project". If I allow others to redistribute my code, I still have to require those others to include this notice. I can't take the OpenBSD project's code and redistribute it under a license that says "you don't have to give any attribution".

However, as the copyright holder, the OpenBSD project can decide to relicense their code under a new license that says "no attribution is required". They can even do so selectively: they can sell you code under this license, but keep distributing the one with attribution required to others. The provenance of a copy of code becomes important in these cases, since different copies may have different license terms attached.

The MySQL project was doing exactly this before the Oracle acquisition, this is not a theoretical idea. You could get the code for free under the terms of the GPLv2 (or 3?), or you could buy a commercial license from them that allowed you to redistribute the binary with any modifications without sharing source code.