The shield generator was completely safe according to plan. They had no reason to ever want to destroy it and put themselves at risk. And by the time the Emperor is long dead everyone on the Death Star seems to be way too busy with something else to worry about suicide bombing any rebels. Who's to say destroying the moon would do any damage to either fleet anyway? For all these people know they would be killing their own legion of troops for no reason. Senseless.
In fact I also seem to recall the rebel strategy for avoiding superlaser fire was to engage the imperial fleet at point blank range. So obviously the imperials do care about friendly fire.