"AI Difficulty has no effect on pirates.
Also those are rebels, and that is a blatant problem, there is no problem with pirates accumulating forces from successful raids, though I agree with NHD that they should actually do something with those forces.
No problem in terms of game-play or eating needless system resources to maintain the literally thousands of pirate units that accumulate at their base? There is NO reason for the pirates to accumulate.
There is no way to use all those pirates for a balanced game play experience.
They came from nothing...they should go back to nothing.
They don't collect resources to grow. They don't cap planets. There are not manufactured in factories...they just appear out of nowhere. If there is a limit cap on players and gravity wells, it's only logical that there should be a limit cap on pirates and rebels.
And anyway, I don't care if there are a million pirates there...as long as they don't slow my game down.
Technically, they do collect resources whenever you or your opponent gives them bounty to attack somebody. When you look at it from this perspective, they don't come from nothing, so why should they go to nothing? Also, yeah, trade ships are a much bigger burden than pirates just sitting around a gravity well.