This is a very good question, and I think there must be a reason this is done, at least in the past (then maybe was carried forward).
I'm wondering if it was to try to keep debris away from the pivot of the knife blade? In the past it could have been because you needed to get at the nail-nick, and then it just kind of stayed, or they wanted it to be the same as the metal file/saw which still requires it the other way (but they could have changed that).
I'm sure it was by design at one point.