The problem is that, for ships that can't outrun or outmaneuver the pirates' speedboats, there really isn't much that armed guards can do. The pirates are generally armed with RPGs-- all they have to do is get close enough, and they can threaten to put a round in the target ship's hull. A sharpshooter might be able to take out the guy with the RPG before he gets a shot off, but what ship owner is going to want to take that risk?
Good point, it does potentially escalate the risk. Still though, a couple dozen sailors spraying machine gun fire in the general direction of the speedboat might give a few pirates pause.
I just hate the "theres nothing we can do" approach to anything, lol.