The military objective is to reduce the enemies ability to wage war. Inflicting military casualties is a legitimate means to that end otherwise any and all weapon use would be a warcrime. Efficacy is not a concern in this regard. In fact ethically speaking, the precision drones allow is an improvement over almost all other weapons (even bullets miss and strike civilians hundreds of meters away).

By continuing to hold a weapon even retreating troops (which these 2 aren't doing in the clip ftr) maintain their status as an immediate threat which makes them fair game, and in the fog of war even throwing down a weapon isn't enough because they likely have a sidearm and/or grenades.