Could it be that the schools requiring their instructors to hold CPLs might want them to teach at CPL level as well as PPL. I'm an instructor on a PPL but am doing my CPL as the ATO I teach at also has commercial students that I can't yet teach, till I've got my own CPL.
Worth checking the training offered by those schools to see if that's the reason, though I agree that if the school only offers PPL training then it would be overkill.