Let's a assume a flight from the US to Europe with two legs: A transatlantic and a short connection inside the EU. The ticket is sold and the transatlantic is operated by the marketing carrier, say Lufthansa. The connection is on a different operating carrier, let's say Aegean.
Now Aegean cancels the connection and the whole itinerary becomes unviable. Who would be required to issue a refund and who should the passenger contact? Looking at the letter of the law it would the "operating carrier" which is Aegean in this case. However, Lufthansa has the customers money and owns the itinerary. In a more complex itinerary involving multiple airlines, only the marketing carrier would be able to untangle the individual fares of the different airlines.
Let's say that the ticket was $1100: $1000 for the transatlantic and $100 for the connection. If Aegean is liable they need to come up with $1100 even though the value of what they sold is only $100. Lufthansa has the $1000 but they haven't done anything wrong, since they didn't cancel anything.
How is this supposed to work?