From what I understand, the commander of the task force was under orders to shoot only if the captain's life was in danger. The pirates put an AK-47 up against his head, and then they shot.
My question is, if they had this capability the whole time, why didn't they take it earlier? What was the purpose/necessity of negotiating with people they did not have to?
Direct orders from the CiC, President Obama.
http://www.slate.com/id/2216003/
President Obama had given standing orders for the Navy to take action if the captain's life was in danger, and as the situation seemed to grow tenser by the minute, Cmdr. Frank Castellano decided he had no choice and gave permission to fire.
With sniper activity, you have to have two things - a green light and the opportunity. Apparently they had the green light subject to certain conditions. As soon as those conditions were achieved, when they had all three remaining pirates clear, the Commander gave the order, and the President's order was fulfilled.
Let no one doubt that sniping is difficult. Sniping from a ship to another ship is amazingly difficult. Waiting for three head shots to present themselves cleanly AND the prisoner to be threatened at that moment - wow.