The root drive is quite a special device for the OS. There are probably tens, if not hundreds, of references to it in the system. It seems likely to me that if you could rename it, you might crash the system badly. These days, though, most drives are identified in /etc/fstab by guids, so I'm not sure that would actually be an issue.
It just seems to met that if the system doesn't allow you to change its name, there's probably a very good reason for it. Linux has always been pretty much a "have it your way" kind of OS.
