I thought the hinges are there to be able to adjust the mirror to the driver's eye position. If you move the seat forward or your upper body is longer the angle at which you look at the mirror changes.
https://youtu.be/JIO7lZuokpQ?t=7m25s
Also, I'm not sure why the OP referred to them as rear view mirrors. I've only heard them called side view mirrors.
For manual-type mirrors the same joint - some kind of ball-and-socket affair, I imagine? - performs both functions. The joint has a much wider range of movement than you'd expect if it were just for catering for the position of the driver's head.