Found this quote interesting:
Please note that as required by the U.S. Department of Transportation regulation, baggage service charges for your entire itinerary are determined by the marketing carrier for the first segment of your itinerary. Your originating marketing carrier is defined as the airline whose flight number is assigned to the first segment of your itinerary. If this carrier is not United or United Express, different charges may apply.
Does this imply that United imposes the baggage fees for the OP's flight?