It has always struck me a slightly squiffy that a guy who stays on the ground can make and overrule the operating Captain, from what I have been told by US pilots.
It'so more of a check and balance system. You wouldn't want us cowboy Yank pilots going hither and yon without adult supervision would you? The Captain always has the power of the parking brake. One of the questions I remember from the dispatcher oral was, "Who cancels a flight?" The Captain can say he's not flying the trip. Scheduling will just round up another pilot. If the Dispatcher says "We can't fly to XWZ this morning" the flight disappears from the schedule.
I've heard this was the crew's first real ETOPS flights. Certainly their first to Hawaii. There will be plenty of blame to go around. The aircraft schedulers built a plan with this airframe going to Hawaii. Dispatch cut paperwork sending the plane to Hawaii. The crew was the last link in the chain.