There's plenty of available heat outside, even when its somewhat cold outside. This breaks down when it gets really cold, like arctic kind of cold, but most places where lots of people live don't get that cold for very long. Its the same principle as how even when its hot outside your AC unit can still manage to dump some heat out there. Its because of the latent heat of the refrigerant and the phase change induced by the changes in pressure from the compressor. Its kind of hacking the physics of phase changes to move energy from one place to another.