phonebook wrote:not so sure- after all i still kill a ship even if it does eject its cargo- and then kill the escape pod too!!!

OK ... individual psychopaths aside, it's in the long-term interests of professional pirates – the kind who hang around in gangs, and presumably have bases of operation and more-or-less local cruising grounds – to encourage traders to co-operate. If they try to slaughter everyone who comes by, then a) trade will drop off, and b) the traders they do catch will fight to the death because they've got nothing to lose. If, however, they extract a toll from passing shipping, then everybody's happy. Well, not happy, but happier than they would be if they were dead, anyway.
Which is not to say that there won't be fights to the death. Not all merchant captains will be prepared to drop their cargo. And sometimes, too, pirates might not be thrown off the scent by, say, ten tons of food, when they think that the ship they're attacking could be carrying 80 tons of luxuries as well. Or if pickings have been thin of late, then pirate captains would be under pressure to make a big score.