[jordigh]

Because you're misreading it. It doesn't say you have to relicense the code. It just says (A)GPL terms have to apply to both parts of the code. That's not relicensing. You don't need anyone's permission for weakly-licensed code to do that. Weakly-licensed code already allows it, that's why it's a weak license.

Weakly-licensed code also allows to be covered by a proprietary license. Nobody needs to relicense weakly-licensed code to lock it up in any way and apply further restrictions to it, whether they be "good" restrictions like the (A)GPL, where you forbid further restrictions, or "bad" restrictions like a EULA, where you forbid people to look at your modifications or to do further modifications.