Just out of curiosity, does it cost money to interline bags with other carriers? If so, I can see a financial (still customer unfriendly) reason behind this.
LAX
If there is an interline agreement in place, there is no specific cost that should be attached to each interline event...I am interpreting the rule to mean the lowest bag cost on the itinerary is the one to be charged....
therefore:
ticket 1 on US - $75 bag fees
ticket 2 on carrier x - $25 bag fees
US would lose the 75 in fees as only the 25 can be collected per the DOT rule - therefore US doesn't want to collect a lower amount and just refuses to push the bags to the other carrier. On a single ticket itinerary, they don't have a choice.
That's my interpretation of the rule and US' response, but I could be way off. I'll be researching the rule further as there are some TA impacts.