Perhaps they just don't like sharks and are eating them out of spite, or the livers are a treat that don't alter their normal feeding schedule, which I imagine involves constant eating of fish.

I'm guessing they're successful hunters and can afford to eat only the choicest bits of their prey.

I saw a good documentary on grizzly bears catching salmon. While the younger, less successful bears will eat any salmon they can catch, the older and more experienced bears can catch so many salmon that they'll just slit them open, eat the roe, and throw the rest of the salmon away, so that they're getting maximum calories per unit of volume in their stomach.

My best guess would be that orcas love eating fatty things like shark livers.