For the command prompt specifically, I believe Microsoft's policy basically boils down to "the correct behavior is whatever it's doing now." It's provided primarily for backwards compatibility, and someone somewhere is relying on every single 'bug' in it, so any fixes they try to make will inevitably break some twenty-year-old process or another.
[The above information is being vaguely recalled from something on Raymond's blog, but if someone has an actual reference it would be greatly appreciated.]