You cant compare our real world to discovery here.
When was the last time a tourist liner was attacked by pirates? And do you really think that they would capture a liner with 2000 western civilians aboard and sell them into slavery?
They dont do it cause they know that so many passengers are very hard to control, that there isnt a market for slaves (espacially for western people cause their nations are were likely to take actions against it) - and yes, these nations would use force to free them. Maybe to prevent another incident like that they fight a hole war (similiar to Afghanistan after 9/11).
As long as pirates capture some transports, killing not too much people and if, then mostly not from a western country and just wanting a few millions, they can do it more or less without too much trouble.
