What's abusive about it? You'd rather he lose more karma from a mistake that he's already learned from?
In general, I'm in favor of people acknowledging past mistakes, rather than pretending that nothing their past selves did was in error. Even when the acknowledgment is of the form "oops, I suppose it was a mistake to voice this opinion in this forum" rather than "my opinion was wrong".
I'd really prefer we didn't have a social norm that changing your mind in response to other people's input is a bad thing, even when sometimes people learn a different lesson than the one we wanted them to. Besides, is it really so much worse for someone to learn the lesson "this sort of thing results in karma loss" vs "that comment didn't contribute to the site and wasn't worth posting"? Are they really such different lessons?
He posted and immediately retracted. The retraction wasn't because he'd learned from his mistake, it was to protect himself from punishment.
Now that he's deleted his comment mine looks asine, as it lacks context. However, I will not retract mine.
http://www.quickmeme.com/Overcoming-bias-guy/popular/1/?upcoming