Well, when you look at how many such pirates were press-ganged in the first place, and how cosmopolitan their experiences and make-up were, it's not surprising; they HAD had walked a mile in the other guy's shoes - or manacles, in this case.  So their natural sympathies were with the cargo, when they took a blackbirder.

That's not to say they embraced them with Sixties™ lovingkindness, singing “We Shall Overcome,” which is what you're intended to think.  There were several occasions where a slave ship was overrun, its crew killed, the slaves freed, and the pirate ship sailed on, leaving a shipload of bewildered Africans who hadn't the slightest idea what an ocean was, let alone how to sail on it.  That wasn't the pirates' problem, was it?  In manu tua, Domine…