The Somali coast is some 1800 miles long. How would someone in a helicopter tell if a boat is in normal trade or fishing, or in illegal enterprise?
Moreover, some reports of the pirate boats being provisioned by vessels that seem to be in peaceful pursuits.

The entire Arabian Sea and northern Indian Ocean, from Africa to Arabia to India, is subject to actions by the pirates.

http://ww.geographicguide.com/africa-maps/indianocean.htm