No, your getting confused, see it's the pirates that were immune to poison (from all the poo eatin'). The ninjas just like to use poison a lot to defeat their foes, cause they're all like that and stuff. Incedentally, this is part of the recent why ninjas cannot defeat pirates. Lesson learned: Ninjas suck and Pirates smell.
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*moves to New Zealand*
And wouldn't their meat alone kill you when you ate it?
'Cause they're stupid.
That and they're immune to it, so they do because they can.