I say it's 'terrorism,' as these pirates instill fear into the ships crew, when they attack.
This might be news, but every hostile act taken by any robber is meant to instill fear. "Terrorism" is frightening hostile action taken for a larger purpose. At least as the people who make these decisions define it - these guys don't have any goals.
I actually do believe what Cthulhu probably said in jest - pirates have a job, to steal things, and the nazy's job is to stop them. That there's violence involved and no formalization doesn't make it any less of a real job. As others have pointed out, Somalia is one of the roughest, poorest countries on Earth with no law or authorities to speak of, jutting into the middle of a major shipping lane, and home to a bunch of desperate, battle-hardened militiamen with guns and boats. How many other outcomes could there be? Accepting the logic of the situation doesn't imply sympathy for the criminals. The "toxic waste and illegal fishing" aspect is news to me, but it's just one more reason for these guys to be angry and violent.
This whole ransom thing is actually a pretty new development, and it's a sign of how utterly ineffective proactive anti-piracy efforts have been. Successful criminals don't boast about their activities, they don't announce their intentions or how they act. Unless they have no reason to think law enforcement can stop them. For decades piracy was a lot more like you'd expect - boarders would storm onto the ship, grab up all the valuables and cashbox, and run. Or they would hijack the ship outright, take it back to port, and sell the hull and all the cargo. Ransoming requires announcing to the world who you are, where you are, and what you're doing, and it's only happening now because the Somali gangs have twigged to the fact that there's just nowhere near enough navy assets to track them down.
It's worth pointing out that ransom works just fine too, and it's already evolved a sense of... decorum. Pirates show up, if they can get on board they call the home company, a few days later a sack of money arrives by parachute, and everyone goes home safe and secure. In the scope of international shipping, a few million dollars here and there for ransom can be written off as an operating expense. It's only when the pirates, crew, or navy bollocks the whole thing up that anyone takes notice.