I thought about this last night while I lay awake. The answer that I came up with from a JW perspective or even from the perspective of a believer in the bible is this: The heavens represent the spirit realm, the earth represents the physical realm. Even though there are many planets and perhaps many species of intelligent life the fact is that the Devil now inhabits the physical realm and only the physical realm. The idea that God would allow him to be here to cause so much trouble can be resolved by thinking from his perspective that any harm or pain done can be undone by him. The stability of 'the big house' or the ruling realm of spirits must be protected at all cost. To disturb that means that the damages done may not be undone in the future.
So basically God is an impotent selfish prick who would rather we suffer because he has the magic bandaid to fix it all but he can't really be arsed to deal with the nuisance of Satan causing trouble with his favorite children the angels or disturbing his naptime. You know that God gets cranky without his naptime