A ninja can become a pirate fairly easily, however, a pirate can never become a ninja.  The laws of physics won't allow it, if one were to try, all of reality as wee know it to be would be threatened, and all pirates and all ninjas, recognizing this dangerous fact, would set aside their differences and put an end to the interloper.

See, pirates are too drunk to properly handle the inherent powers of the ninja.  They would become unbalanced, losing all focus, and thus become an unwitting conduit to the hoards of souls lost to the ninja, unleashing literal hell on earth.  Pray that all pirates never lose site of this simple fact.