This is what I found after learning of the changes to MSC:

In July 2012, the United States Department of Transportation (US DoT) Rule 399.87 came into effect. Under this rule, all carriers selling transportation to passengers, whose ultimate ticketed origin or destination is in the United States, must apply the same baggage policy and fees throughout a passenger’s journey, regardless of the number of baggage check-ins during the journey.

Accordingly, the US DoT requirements stipulate that it is only the first marketing carrier on the first flight segment of an itinerary that has the right to establish the baggage rules to apply for the entire journey. This is irrespective of stopovers or other carrier flights listed on the single ticket. More specifically, the first marketing carrier has the right to choose to apply its baggage rules, the rules of the MSC, or those of any other carrier on the single ticket. In case of itineraries where the first flight is operated on a code-share basis, the rules of the marketing carrier must apply.

All carriers must reflect their baggage rules in their tariffs filed with the US DoT.