Does Cambridge University Press Ban Chapters Authored Solely by Grad Students?

A graduate student in philosophy wrote to share that he and another student had been recently booted from an edited collection under contract with Cambridge University Press (CUP) because the press does not allow chapters to be authored solely by graduate students.
The policy was apparently unknown to the volume’s editor, who had included the students among the contributing authors in the book’s proposal. According to the student who wrote me, the editor conveyed the news to him after hearing from CUP about the rule, which was justified on commercial grounds—that including chapters authored solely by graduate students may deter some potential buyers.

Do any other presses have such a policy?
I reached out to an editor at CUP to determine whether in fact this was a policy at the press, but have yet to hear back. If it is a policy, it would have to be relatively new, as some looking into the matter revealed that the publisher has in the past released collections which contain some chapters authored solely by graduate students.
Discussion welcome.
A blanket ban on graduate students seems like a mistake on editorial grounds, as sometimes graduate students may be the best prepared to write on specific topics. And while I’m no marketing expert, the commercial justification for it seems to be a stretch; I’d be shocked if anyone responsible for this policy presented data that suggested that, say, an edited collection with a chapter or two written by graduate students will sell less than one without any graduate students.