I've seen a lot of discussion on this in some of the Navy-themed blogs I look at (like EagleSpeak (http://www.eaglespeak.us/2008/09/somalia-captured-pirates-set-free.html) and Information Dissemination (http://informationdissemination.blogspot.com/search/label/Somalia)). It does sound like the issue is a little more complicated than you're portraying it; the "motherships" generally seem to be fishing trawlers, and these are some prime fishing grounds. You generally find out that a ship has been taken when the ransom demand is received; the victims seem to often be either under-crewed rustbuckets, or yachts that happen to cruise through.

There's also the question of proving the guilt of whatever individuals get snapped up, unless they're caught in the act; it's also generally desirable to not get the hostages killed. Finally, I'd expect there to be some concern over the PR of "US military shoots captured Muslims without trial".

In truth, I think what we're seeing here is a case where, since it's not US ships getting taken, the US Navy has been directed to not expend too much effort in dealing with the problem. This may be intended to demonstrate to others the folly of expecting the US to be the world's policeman and not providing adequate support... but it's not going to come up in a candidate debate. I'm trying to imagine Sarah Palin answering a question on the subject, for instance.