Characters act against their own interests in HPMoR... and in Hikaru no Go, for that matter. Just, you get to see why they're doing it so it seems more reasonable at the time. Which is of course how it seems to them. We're just used to characters doing things that don't seem like good ideas at the time in other works.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.