In a typical RSS reader, the relationship between the user and the content producer is more pronounced; the user proactively selects content sources and knows where each piece of content comes from; as a result, there is still a significant element of brand reinforcement.
The internet has discussed the economic implications of RSS readers ad nauseam so I wont go there. However, this isn't just an RSS reader; this site allows me (as a reader) to consume an amazing amount of content with pretty much no idea where it came from and very little inducement for follow-up interaction with the content creator.
In other words: Google Play is basically stumbleupon, except in Google's version, the "toolbar" takes up the entire screen. Google is redefining the transaction in a way that minimizes value for the content creators and maximizes value for Google.