Didn't read the PDF, but I suspect the main problem is that not all "communities with downvotes" are the same. Some websites have downvotes where the downvoted comments are just as visible as the upvoted ones; so there is no punishment.

Not sure if the causality can't go the other way round: crazy people having more time to post comments. Or even that members are more tolerant to new crazy people and annoyed by them later (which causes the decrease of comment karma), while the crazy people gradually write more and more comments because they become more comfortable or have more open battles to fight.