Atwood's an American. Writers will write what they know… they're hardly obligated to use an unfamiliar measurement system for your convenience.
I note that the date of his blog post is the international-friendly “November 19, 2012” rather than the American numeric “11/19/2012”. But internationalising dates like this is easier than providing measurements in dual units.
It's almost like MM/DD/YY is the abbreviation of "Month DD, YYYY"
01/02/03 can be January 2nd or February 1st, and often there's not enough context to determine which was meant.
That, and also Atwood's popularity among American readers gives him a great opportunity to encourage them to use metric.
In the meantime, use http://news.ycombinator.com/item?id=4751410 :).