All deraileurs have a return spring - some shifters do not (GripShifts and basic friction thumbshifters for example).

To the OP -

Top-mount thumbshifters are, im my opinion, the best thing since sliced bread. Ironically it is a workaround to make a product similar to the old-fashioned shifter style. THe primary advantages are much easier maintenance (as opposed to completely unable to service like rapidfire) and the ability to select 'friction' shifting mode if the indexing is not working properly. And front shifting is friction all the time, which is really a better way to shift.