From the little I know of Buddhism, I think the reason that the monks pick the direct route of pushing the fat man is intention.
In Buddhism, there's a story of a man who killed the captain of a ship because the captain was leading the ship into danger that would kill all the men. As long as the man's intention was to save the crew, then killing the captain wasn't seen as bad.
Using the switch to divert the trolley would be deception, which is not a pure intention.
The idea is that if you're going to do a wrong thing, then don't disguise it by trying to deceive others you did it because that's not a pure intention.
My question would be whether you can program intention, and more importantly, whether that would be a good thing. Perhaps not, because humans having a pure intention is a good thing because it's clear who is taking responsibility. If the car is doing the killing, is it the car (or the owner of the car) that takes responsibility?