Keeping the peace... [Small pause. Weird idea? Well, not really, after all, keeping islands safe from all the other pirates and chaos and death of the Grand Line was exactly what Marco's family did so well. But you know... not in a legal way anymore than gangs defending their streets.]

Well, they the marines are a big part of the reason so many pirates die before reaching the end, eh. That's their job. They don't want another pirate King on their hands.