Why? Because correlation always implies causation? How do they even know he was stoned at the time of the accident?In the case of the train, the engineer ignored the speed limit and blew through a red light. Would that have happened if he had not been high on pot? We don't know. But it has to be held up as a strong possibility.
If you don't know the answers to these questions after all that has been posted in this thread thus far, I doubt anything I could say further would make one whits bit of difference here. So I'll such refer you to read the thread again. If you don't find your answer, well. . . .