It also suggests an explanation applicabloe to both. The machsheifah wore
sha'atnez for a reason -- kila'im is a subversion of nature, an integral
part of her prohibited art. Perhaps the Rambam was saying the same. It's
not assur merely because that was what the Kina'anim did, but rather it's
assur for the reasons the Kina'anim wanted to do it.
http://www.aishdas.org/avodah/vol05/v05n077.shtml