I'd guess that the iSight mirrors things because when people are viewing themselves on a webcam they view it as they would if they were looking in a mirror, so if for example you need to move to centre yourself you'd move the wrong way if it wasn't mirrored. It just makes it easier to position yourself properly. Folk who you're chatting to see the image unmirrored though, I think I'm right in saying.
The iSight's default setting for photos matches the default webcam setting, hence the mirroring.
That might not be the reason but that would be my guess. 😉