The (A)GPL explicitly allows certain restrictions under section 7 (Additional Terms).

Back in the day FlowPlayer (a JavaScript component to play videos on the web) used additional terms per GPL section 7 to force users of their unpaid version to keep their mark intact in the UI or for modifications keep a "based on FlowPlayer" in.

I followed the discussion on this back then and the consensus seemed to be, that this is in line with what section 7 of the GPL allows. I think there was even a statement of the FSF, but I could not find it anymore.

Of course the iText case is different, but I believe section 7 (especially 7b) allows a way to add terms for attribution.