SAS can at any time book a ticket on any airline that is a member of IATA for whatever reason. There are IATA agreeements that cover this. You can even go to as SAS ticket office and buy a regular ticket on their competitors, like British Airways and Air France. That ticket is just as binding as e.g. a ticket booked through Expedia. Expedia can also at any time book a ticket on any IATA airline.
An airline cannot refuse to transport a passenger because it is booked on the "wrong" ticket stock. That would be a violation of the IATA membership rules, and a violation of EU 261.
SAS will naturally have to pay the airline for this, so I don't understand why any airline would even think of refusing to accept the passenger. Why fly with an empty seat if SAS will pay for that seat through IATA?