The difference is there are no hypothetical fat men who are near train lines.
What are those thought experiments good for if there are no real-world approximations where they might be useful? What do you expect, absolute certainty? Sometimes consequentialist actions have to be made under uncertainty if the scope of the negative utility involved does outweigh it easily...do you disagree with this?
The problem is, as has been pointed out many times in this thread already, threefold. Firstly, we do not have perfect information, and nor do our brains operate perfectly - the chances of us knowing for sure that there is no way to stop unfriendly AI other than killing someone are so small they can be discounted. The chances of someone believing that to be the case while it's not true are significantly higher.
Secondly, even if it's just being treated as a (thoroughly unpleasant) thought experiment here, there are people who have received death threats as a...
It's probably easier to build an uncaring AI than a friendly one. So, if we assume that someone, somewhere is trying to build an AI without solving friendliness, that person will probably finish before someone who's trying to build a friendly AI.
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further edit:
Wow, this is getting a rather stronger reaction than I'd anticipated. Clarification: I'm not suggesting practical measures that should be implemented. Jeez. I'm deep in an armchair, thinking about a problem that (for the moment) looks very hypothetical.
For future reference, how should I have gone about asking this question without seeming like I want to mobilize the Turing Police?