If I was to guess, the thinking behind this was now that they have windows 7 growing in market share and a few households upgraded, they can use word of mouth advertising to a greater extent. If the family down the street is saying how amazing, safe, and easy to use windows 7 is, then before you know it the rest of the block is looking to upgrade, regardless if they can get the family back or buy individual ones.
Now whether or not that actually happens is up in the air, but I'm assuming that's what MS is going for.