And the IATA rules dictate that they must interline bags from carrier to carrier if the booking is all on one ticket.
A couple of months ago there was someone being checked in next to me in MAN flying on to KUL on Malaysian, connecting from BA to MA at heathrow. The Check-in agent said to the couple that she could only check bags as far as heathrow. I heard this, turned to the couple and asked if it was all on one ticket, to which they replied yes. I just said to the agent that IATA rules dictate that you HAVE to check the luggage through. She wouldn't believe me and I suggested she speak to her supervisor. She came back and checked them through. The couple thanked me graciously.