It's a question that could also be applied to any capo where the open side remains open when clamped, like a Shubb or the ubiquitous spring capos.

I have always fitted them open side downwards on the assumption that the clamping will be better near the closed end than near the tip, and that the bass strings will need more clamping force than the treble strings.

On the other hand, open side up seems more natural, as it mimics the hand.

I've often wondered what was the intent of the designers, but not enough to do anything about it.