Butter is refused because, once it is taken out of the buttermilk, minute living beings arise in it; ghee, which is butter cooked, is not refused.
The handbooks say beings arise in fresh butter within an antarmuhūrta of its separation, which is why the same milk is fine as ghee and not as butter: the cooking ends it. It is the one of the four vikṛtis that is not a ma-kāra and not among the Digambara mūla-guṇas, which is why some lists count only three great vikṛtis and keep butter as an abhakṣya on its own.
The twenty-two, laid by their reasons - Butter highlighted