They restrict the usage because.. they want to control how devices behave in their network. They want to be able to do network load management, not let users mess with the bands.
Also, when Samsung flashes a specific CSC radio, some bands, that are not in use by that provider, get turned off. That saves power and possibly improves network connectivity.
My factory unlocked U1 version has multiple CSC in the firmware that can be automatically applied.
Today, we'll explain Samsung CSC codes. Besides, we also have a list of CSC codes or region codes with a method to change CSC on Samsung.
Those are my educated guesses, the providers can have their own reasons. And if we don't like it, we can choose a different provider...