But that's the thing: it doesn't. You can take GPL'd code, modify it, give your derivative work to other people and never contribute back.

You don't have to "give back" (push your changes back upstream), but you do need to put your changes out there to comply with the license once you begin distribution.

Not "out there," just to give the source to the receivers of binaries.

I know that in the context of B2C that sounds like an irrelevant distinction, but in B2B, it isn't - such software is often distributed only to a handful of organizations, who have no interest in becoming redistributers even if they have the right to do so.

I can tell you that there are lots of derivatives of copyleft software that are never made available to the original developers, in a perfectly legal way, and people who are choosing a license for their code should realize that.