The following may have no connection to the real reasons for this; but even without Many-Worlds you have a non-zero probability of personally suffering any possible punishment. Legally allowing a given punishment for anyone seems to produce a non-zero increase in this probability (even in a world without Polyjuice).
On the other hand, reducing the deterrent for potential dark lords, increases your probability of winding up living under a dark lord at which point your chances of suffering horrific torture, either in Azkaban or somewhere else, is greatly increased. Assuming you don't consider being wrongly punished in Azkaban under the current administration vastly worse then being punished and/or tortured under a dark lord, you can't simply declare certain punishments out of bounds.
Another way to think of this is that any government that fails to provide sufficient deterrent to prevent successful overthrows will be overthrown. This process will continue until you get someone who is willing to be sufficiently brutal. So it doesn't matter how nice your ideal government would be; if it can't prevent overthrows, you won't get to live under it.
That certainly seems like the relevant Star Goat probability. (I speak of the One True Star Goat, braise His mane, who will devour the souls of all who believe in God and make them stew in His Holy Bile for eternity, not the vile worship-demanding blasphemy proposed by the Restored Church of the Star Goat.) The Anti-Pascal's Wager argument may not work here, though.
The part of your argument that deals with Dark Lords overthrowing each other until we reach sufficient SP assumes that some possible deterrent will stop them -- although canon!Voldemort clearly ...
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.