I heard that the film crew had wanted so desperately to get all of their gear with them back to wherever that they were going, that they paid off the pilot to carry their gear with them on the plane. They figured that if they could fit it in the cabin, it wouldn't be a problem. However, that made the plane about 500 pounds overweight, so it went into the ground. It was just a boneheaded mistake on the crew's part and an apathetic pilot that got them all killed.
Second, the airline industry didn't screen the pilot because he wasn't in an airline. I order to save money (cheap bastards) they hired the pilot for a company that does tour flights to fly them. What they were doing was illegal.