For a significant percentage of the participants in those studies, reading the counter claim increased their belief in the original claim. OK, my previous post outlined why it might have no effect, but HOW can it possibly increase the original belief? The same way we (over) react to a lot of the things we disagree with or don’t like. The process of counter-arguing makes us think hard about how the original claim could still be correct. This strengthens the schema, even without any new evidence in its favor.
Or maybe just to spite the source of this horrible news (in true "shoot the messenger" fashion), we overreact and believe the original even more. "How dare you tell me that my favorite candidate or pet policy is not as good as I thought ?!?! Just for that, I will support it even more!!