Or conversely, why don't the users who want to customize the prompt set up special user accounts to do so?
You cannot create any accounts other than the existing root account and be able to talk to the power pmac. I tried setting up another account and without wrapping the entire gpascii there is no way to communicate to the Power pmac with an account other than the factory default root account. Try it!
I agree with what's been said that the Power PMAC is a powerful motion controller that just happens to be running on an embedded Linux OS. I enjoy the openness that DT has provided with the control but clearly they can expand on that to give end users more "rope to hang themselves" or freedom, however you see it. ;-)
Anything DT does to cripple the system is like giving someone a sports car and installing a throttle limiter. Not nearly as fun, but I will still take one!