The cruise lines aren't the ones with the problem, it's the cargo ships. The pirates only want to rob the crew and empty the ships' safe. The shipping companies won't spend the money for proper protection, so the crew is often left vulnerable to piracy. The varying international gun laws also play a huge part, in favor of the pirates. The shipping companies don't want to run afoul of local law and risk having one of their vessels, along with it's cargo, impounded, so they're not armed.

So far, it looks like the Russians have been the only ones to take a stand against piracy. A few years ago, several bodies were found floating off the coast of Malaysia. The next day, a small boat, outfitted with some serious firepower (as well as a few more bodies), was found not far away. Although it was never proved, it appears that these were pirates who were attempting to board a Russian freighter off the coast of Sumatra.

Check my previous post in this thread, and if you get a chance, read the book.