The USB ports have no control over the power. In all implementations I am aware of, the 5v is just hardwired to the port as it is with the PS/2 Keyboard / Mouse port. Unplugging a device will not affect the power of other devices on the same bus. They simply keep drawing the same current they always draw. You can't force-feed power to something with a regulated supply.
The USB controller simply gets, as part of the setup info from the device, an indication of the power required by the device and tracks what is being used against what is available. Based on that information, the controllers supposedly will not enumerate a new device if it thinks more power will be drawn from the port than is available, but I have yet to see this condition occur. Maybe I just haven't tried hard enough

In any case, as others have already stated, HID classed devices like mice and keyboards were absolutely designed to just have the plug pulled on them. But anything that stores info, even in non-volatile memory, should be shut down in the OS before disconnecting. I've seen very bad things happen under 98 when not doing so, and while I'm sure 2K/XP is likely to be safer in this regard, it's still not safe for the device being disconnected.
RandyT