The same reason there is a law against vigilante justice. In many individual cases it's probably ethically justified but I certainly support a general rule against it. Because I don't trust the judgement of all those other f@#$s so take the cooperative mutual suppression of the behavior as the best option.

This is a strong point and perhaps I was undervaluing it. But I wonder why 'not having sockpuppets' stands out as a rule that's so important to uphold and start talking about banning in the name of. It doesn't seem to have actually caused many problems on LessWrong, and in fact I suspect that not overly discouraging sockpuppets has had a net positive effect, as it has allowed some people to make interesting posts they otherwise wouldn't have. Of course it has also allowed for some boring people to be boring, so it's not an obvious question, but its not being obvious also means that talking about banning people for it is in my opinion pretty weird. But maybe you've seen sockpuppets become a severe problem on other fora or something? LessWrong is the only forum I have a decent understanding of, but I do think that after many years of painstaking engagement my understanding is rather decent.