Somebody else posted a thread like this a month or so ago.  As you stated, precision sniping from a moving (or even hovering) helicopter would be extremely difficult.  Another factor would be that the sniper would probably have to fire from above the target, which would affect the bullet's trajectory and impact point.  Still, it is not terribly uncommon to deploy snipers on helicopters to provide precise area support for ground troops.  One agency that deploys snipers in helicopters very successfully and efficiently is the US Coast Guard, which deploys .50 cailber rifles in its UH-60s for use against fast boats.  They aren't used to take out dreg smugglers per se, but rather the engine of the boats to stop them dead in the water. 

 

The UAV question is an interesting one.  I suppose it would be possible to fit a gun of some sort to a UAV to deploy against enemy targets, but you have to look at the practicality.  Hellfires give you a better chance of taking out an individual target in a car or a house, but there's still the risk of collateral damage.  The challenges would be deploying a gun equipped UAV that could fly around at several thousand feet, yet still be stable enough and have a good enough weapons system to accurately engage a man sized target.  I guess its doable, but it may take a while, and it may not be worth the trouble. 

It works, though there are definitely stability issues (and ideally some need to train with the helo crew prior to going hot) and other considerations.