Most of shark attacks on surfers are mistakes, so perhaps orcas are just smarter and see better, and can make a distinction between seals and humans on boards? But then again I've heard about orcas sinking sailing boats and trying to attack polar explorers by breaking the ice below them, so it's not like they're completely uninterested in attacking humans. It probably depends how much contact with humans they have. Being predators, it's only logical that they will eat anything they can catch when the hunger strikes...