From another perspective, Microsoft is probably taking some measures to reduce the amount of shovelware that appears on XBLA, and prevent the catalog from getting "too large". Granted sales don't always measure "quality" of some titles (ie. okami), but it's a pretty good marker for their audience. From an economics standpoint, it's a metric they can track pretty easily. Especially since XBLA provides a demo for almost every game they publish (which, provided the user downloaded the demo, correlates it's perceived quality pretty well to the sale)