It doesn't bother me when anyone claims this but I've never understood the rationale behind it.
Let's say that there are (verbally) intelligent, sentient beings from Mars (I'll call them 'red' people) who come to Earth to purchase slaves. Iraq invades America and their captured war prisoners are sold to the red Martians (Iraqi criminals and Iraqis with long-standing debts, who might have already been slaves in Iraq, are also sold, some humans are kidnapped by the red Martians but the majority are sold by other humans for the previously mentioned reasons). China invades Brazil and their prisoners of war are sold to the red Martians, Nigeria raids Tanzania for slaves and the captured are sold to the red people etc. When the slaves arrive on Mars, they are forbidden to preserve any ties to any of the human cultures they came from, they are given Martian names, made to practice the Martian religion, speak Martian languages etc. As a result, over time, they develop new cultures that are distinct from both their ancestors Earth cultures and the native red Martian cultures. How does it make sense for these Martian humans ('Martian' because they were born on Mars, although not 'red' since their species is still human) to argue that Earthlings sold them into slavery when their enslaved ancestors were themselves Earthlings? To argue that Earthlings sold their ancestors into slavery implies that their ancestors sold themselves into slavery since their ancestors were themselves Earthlings (and not 'Earthling-Martian' since the human cultures that were developed on Mars had not yet been created). Their resenting Earthlings for having sold other Earthlings into slavery makes no sense because the people they resent Earthlings on behalf of were Earthlings themselves. Imagine one of the slaves escaping back to Earth and telling his cousin, friend, brother etc. "you Earthlings" (not you Iraq's, Chinese, Nigerians etc., or distinct individuals, but 'you Earthlings') sold me into slavery. It's not any less likely that any Earthling the Martian descendant of human slaves comes across is a distant cousin than it is that (s)he descends from someone who sold their ancestor into slavery.
What am I missing?