I think they have an oral history that goes back hundreds of years the same as Native tribes had before Europeans arrived. I think killing humans is 'taboo' in their culture and they basically don't do it for the same reason certain tribes wouldn't climb certain mountains or drink the water of certain streams. They kill for sport and play with/torture their prey before killing it. They have plenty of opportunity to kill humans, but they don't (outside of Sea World).
I was working as a dive tender in the sea cucumber fishery when one of the divers came up faster than he should have looking terrified. He told me 14 orcas had swam past him and the other diver closely. The other diver finished his dive then came up. The diver that had panic ascended asked the other diver if he had seen the orcas and he replied 'They have never attacked a human'. The panicked diver said he didn't intend on being the first one .