I don't necessarily object to this policy but find it troubling that you can't give a better reason for not discussing violence being a good idea than PR.

I would find this troubling if it were true, but the better reason is right there in the post: "Talking about such violence makes that violence more probable".

I appreciate the honesty of it. No one here is going to enact any of these thought experiments in real life. The likely worst outcome is to off-put potential SI donors. It must be hard enough to secure funding for a fanfic-writing apocalypse cult; prepending violent onto that description isn't going to loosen up many wallets.

If the violence is a bad idea, which in nearly all cases it probably would be, other commenters are likely to point that out. Having people inspired to carry out acts of violence in spite of other members pointing out that it's unlikely to bear good results is possible, but unlikely, whereas having people judge the community negatively for discussing such things at all is considerably more likely.