Probably because on standard Unix operating systems, each user has its own shell, and root is just another user. Mac OS X is more or less Unix-derived. So, lacking any sufficiently powerful reason to change that, it's not surprising that Mac OS X follows Unix conventions.
And it would be very surprising if changing
bob's login shell also changed
root's login shell. That'd be a security risk (ordinary users shouldn't be able to change root's login shell), could cause system stability problems (if you choose a bad login shell or one that is on an external partition), and would violate the principle of least surprise.
Also, probabilistically speaking, I suspect that most users who know enough to log in as root, also know enough to be able to change root's shell to whatever you prefer.
But only Apple can tell you for sure why they did it this way. The rest of us can only speculative.