I have heard from ex-Bethelites on this forum who were there in the 70's that Knorr had a thing against anything denim and would go off on someone if he saw it. He did this without discretion. A Bethelite could be headed for a BBQ at the weekend and this still was inapropriate for him.
This is likely inherited from that era. Just like beards are still looked down upon because Rutherford was clean-shaven and he saw them as a homage to Russell.