If this was not the case, we could basically generate arbitrary amounts of power from nowhere, as there was no resistance and we could thus just spin a generator forever with zero power necessary (except for the little bit required to overcome friction) once it is set in motion. The Lorentz force makes sure that's not the case by slowing down the movement, and it slows it down more if more electric power is pulled from the generator.
If this was not the case, we could basically generate arbitrary amounts of power from nowhere, as there was no resistance and we could thus just spin a generator forever with zero power necessary (except for the little bit required to overcome friction) once it is set in motion. The Lorentz force makes sure that's not the case by slowing down the movement, and it slows it down more if more electric power is pulled from the generator.