Bitmain was the main of the very few supporters of BCH before and after the fork.

I read or heard about somewhere [rumors] that they are trying to centralize BCH by controlling (They sort of already do), so by accepting only BCH, they are sort of increasing demand on it by forcing buyers to buy BCH so they can buy Ant miners. At the same time, they would evetually end up with most of the market capital allowing them to make it scarce when they want to increase it's price, and abundant when they want to lower it's price... Not sure how accurate or true those rumors are, but it makes some sense to me... A lot of the BCH haters also hate Bitmin for that...