As far as I know it's to discourage people from aiming for negative karma, and taking pride in how much they've been downvoted - as proof of their trolling or brave thinking out of the box or willingness to "tell truth to power" or whatever. I think that happened a few times.
(it's a bit like how sometimes punishing a misbehaving child can actually be "rewarding him with attention", and result in more frequent misbehaving)
Interesting. Everything is a tradeoff. When people aren't alerted to a troll, there's a cost, and when they are, there's another cost.
Is there any mechanism for booting a troll?
It came to my attention that when you receive downvotes for your comments, your karma goes negative and you need to "pay back" to be able to post to Discussion or to Main.
Since new users start with zero karma, having negative karma seems to just encourage those with negative karma to create a new account. We don't want to encourage people to create superfluous accounts, do we? Therefore I think LessWrong codebase should be patched so that karma does not go below zero even with lots of downvotes.
What do you think?