Wouldn't it be smarter for pirates to go after the one piece as merchants, fismen or something instead of making it obvious and defying the marines?
black sails decrease their chances of success
Powerfull crew of merchants sailing the seas, defeating pirates.
marines thinking of you as allies therefore leaving you alones.
Remember the strong guys form galeyna company (water seven), they could got after one piece and not be bothered by marines.
I know that means less battle experience, but with a lot of training they would be able to keep up