The Trolley Car Dilemma is False

I was reading a review in the recent NYRB of a book by the man who invented the "trolley problem," Joshua Greene, which shows that many people refuse to kill one man, even though it would save 5 people. A runaway trolley car containing 5 passengers can be diverted if you push a man off a bridge into its path. If you don't do that, the car will crash and kill the passengers. What do you do? 

The "trolley car" problem has become a widely used thought experiment in philosophy classes. For example, Michael Sandel starts his MOOC course on justice with it.    Greene talks about the psychological difference between active agency and passivity. But I think the setup is falsely rigged. The problem presents the deaths as a certainty, a "fact." But anyone in the position of the man who has to kill to save the 5 would not see it as a certainty, but just as a possibility. Consequently, the trolley problem poses a false dilemma.