So far, dominators are shown to have only four enforcement mode. One is Non-Lethal Paralyzer used on targets with crime coefficient between 100 and 300. The other one is Lethal Eliminator used only for human targets with crime coefficient above 300. The third and rarely used mode is the Destroy Decomposer as mentioned by senshin and so far only used on non-human targets. Fourth is shown in movie only, called Explosive Destroy Decomposer.
Dominators have the Lethal Eliminator mode activated when pointed at drones because drones are non-living beings, therefore there is no need to use the Non-Lethal Paralyzer as it would be just a waste of ammo. Also since before activation Dominator scans the user to ensure its user's mental stability and that the user has the appropriate rights, pointing Dominator at drones or other non-living targets means that the user intents to destroy the target, which is why Lethal Eliminator or other lethal shots mode are assumed.
Dominators use destroy decomposer mode on drones mainly because the Lethal Eliminator mode makes the target grow bubbles and explode, which might be dangerous when applied to drones. There are two dangers, one is the explosion itself, since some sharp particles might fly out, and two is because only parts surrounding the shot will explode, which means some part of the drone might survive the shot. The left part might still function, which is why destroy decomposer is used to make sure that the target drone is completely eliminated.