Just a theory, but I think the reason Microsoft renamed AntiSpyware to Defender is to make the name more generic. This way they have the option of tossing anti-virus in if the paid anti-virus service they have does not pay off. Which it probably will not.
By starting with a paid service they can ease into the anti-virus market instead of crashing in. From the technical side, this will give them a smaller user base to work with until they are confident the service is solid.