The answer is simple: the US restrictions on doing business with Cuba. Any company doing business with Cuba cannot do business with the US. By operating the Havana flight via another company, IB complies with US law and avoids any complications. A similar arrangement was used for Taiwan when mainland china refused airlines serving TPE to land in China (lots of separate companies set up, called "xxx Asia", which only served Taiwan). As an aside, any ticket with a Cuba sector cannot be honoured by any airline in the US (and becomes worthless).