Western atheism and antitheism often exist as reactions to Christianity on a factual or ethical basis, while remaining within a Christian cultural and moral context. Historically, Christians and atheists have worked together to attack other religions, such as in the case of the stolen generations in Australia or the cult panic in America. This pattern of behaviour is counterintuitive if you're culturally Christian (as most atheists are), but from a broader cultural perspective it's kind of obvious. Christianity and white atheism exist as offshoots of the same history in the same way that Catholicism and Protestantism do.
If you have an actual understanding of pre-roman polytheism, then you're capable of seeing the difference between belief and worship. A difference Christians have tried to erase, and white atheists have not challenged. Giving up cultural Christianity is beyond most white atheists' ability to even imagine. What I call cultural Christianity, they would just as soon call "common sense" or "reality".
Also I've never met an antitheist who has spent longer than an hour thinking about whether they're advocating cultural genocide of indigenous people.