I think it's because it needs a voltage signal to know at what charge state is the battery. When the battery is dead or there is no battery it doesn't charge i.e. because no battery status to know how much voltage to regulate, running it without a battery won't damage it because the DC is only used for charging the battery. AC for the lights goes from different coil to lights and returns to regulator to be rectified to DC. Even if you revv it to red line it doesn't matter if you have battery or not it won't damage the diodes because it will disapate as heat on the regulator body.Now I have installed a LED headlight and I converted the AC light system to DC. And even if I turn the lights now it won't fry anything because the coils produce more than 10 amps and the LED headlights draws less than 10.
Thanks for the answers I found the cause.