I think of it like this... If you're actually flat basing, you're not actually snowboarding, what you're doing is standing on a snowboard. My 2 year old can stand on a snowboard (flat-base down a hill), but he sure as shit can't snowboard.
May as well stand on a sled.
So you don't consider boardercross to be snowboarding, ok.
Of course no one is talking about going top to bottom in a straight line while flat, but there is a ton of flat base riding in boardercross.