"Heaven" here refers to the celestial beings who dwell in between Heaven of the Four Heavenly Kings and the Trayistrimsa Heaven and those who dwell on the Heavenly ground in the Desire Realm.  Except for Buddhism, most religions teach people to go to Heaven as their ultimate aim.  But the celestial beings also possess desires.  They greatly indulge in the heavenly enjoyment.  If they do not cultivate continuously, they would surely be controlled by the force of karma and fall down into the human realm or even into the hells when the five signs of   decay appear.

   The Heavenly Beings are still in the mundane state as they are not free from the wheels of transmigration.  They have not severed the roots of birth and death.