It sounds like the policy is inconsistently applied or not well understood by the staff.

In my situation, I flew on a K "light fare" (LH ticket stock, LX metal) with no upgrades/vouchers into business. I arrived at the Star Gold checkin line at CDG, gave a polite "Bonjour", and loaded my bag onto the checkin machine. My luggage was checked in with no charges, questions, or discussion about "goodwill". Very easy process, but it sounds like I may have been lucky.

As far as online documentation, the Swiss website booking indicated that I needed to pay, but it sounds like this is also what the LH policy is (you have to pay online, but if you arrive at the gate it will be waived for "goodwill").

On the outbound UA leg of my journey, there was also no charge for luggage. And when I looked at the UA e-receipt, it indicated $0 charges in both directions for the first bag.