In my mind, it is one of two reasons:
1. Someone is hoping to make a profit someday by appearing with a bunch of the original registers, and offering them to a museum for a profit. (I doubt this is the case).
2. Some self-righteous "leave-no-trace" freak with an attitude believes he is doing the world a great favor by "cleaning up" the wilderness. I suspect this is quite likely the case, based on the experience of people like Bob R, where the register on Thor Peak has been repeatedly removed.
