It's all pretty simple and boils down to relative profit margins. Whatever people might think, the actual operating profit to a Wal-Mart or Best Buy on PC hardware is not more than 2-3%, i.e. about $15 maximum for the majority of PCs sold. The profit margin on a $40 recovery CD is pretty much $40.
Therefore, they should push the recovery CD as strongly as possible, until the point where they are so aggressive that they are losing 1 in 3 PC sales due SOLELY to their pushing of the CD. They haven't reached that point yet.