The use of drones means shoot to kill is the only option.  Using drones cuts down the options on the ground. A drone can only keep circling or go in for the attack; their only option is to kill a target or let them escape. Using ground forces; either your own, or in this case the Pakistani or Yemeni military, provides the third option of capturing the target. This is ethically a much better position for the United States to be in as it means that the terrorists can be given the option to surrender rather than simply being killed. This in turn would provide the benefit of allowing a trial helping to show the justice of the operation. Moreover these captured militants would likely be valuable intelligence assets who could be questioned which may well lead the intelligence services to other terrorists.  Finally using drones is a very aggressive and provocative stance as it prevents any possibility of a peaceful resolution. The usage of drones immediately eliminates the possibility of negotiation because drones are remote from their operators. This means that drones are unlikely to be useful in the many situations that could be helped by any form of contact.