But capitalization is not a social convention, but a grammatical one. Take it up with the MLA.
I highly doubt that the MLA Style Manual makes the same recommendation as Williamson does.
Rather, most sources recommend lowercase for both
black and
white, or uppercase for both
Black and
White , but none specify a usage where you would find one capitalised and not the other.
The capitalisation of the black and the non-capitalisation of white, both as stipulated by Williamson and as practised by the morons on feministing, is probably based on a fear of disrespecting black Americans rather than a desire to follow broader style conventions.
It's not an outrage: it just looks silly. And you are defending their silliness.