In this era of highly targeted advertising when companies charge more to put your ads in front of highly specific people groups and demographics, companies like X can easily keep your ads from being associated with white supremacists… they just don’t want to.

X knows it’s popular with these hate groups and they want to monetize them. They’re making money by placing eyeballs on an ad. They’re just hoping no one notices that they’re including hate groups in the eyeballs they’re charging you for. If they can’t display ads to the hate groups then they might actually have to stop allowing the content and they really don’t want to have to do that.