It is similar to Google pushing its own services like Maps and Music results in Search. Also, Amazon may think that users looking for physical goods will stop and get enticed to its now impressive collection of digital stores.
They know that being early to market is insanely important today, even a year delay can end up being the difference between Groupon and Google Deals. It's typically impossible to compete with an entrenched product by one of the big tech corps as they have the engineering man power to duplicate any improvement you make over their formula if you start depriving them of the tiniest portion of their user base.