The only thing I can think of is that reviewing or purchasing through ITMS requires more active engagement, instead of the more passive scrobbling. Of course this engagement requires more effort, but they want people to purchase through iTunes and review things in ITMS anyway, perhaps they like to provide an extra incentive to do so, and at the same time make you more loyal to ITMS instead of other competitive music stores online?

I'm not sure I would've made the same decision (realizing that people are lazy by nature and want their music profile to be built automatically), but these are the only arguments I can think of.