Google has a history of strong-arming people into it's services. And I don't like it.
Basically, reviews are part of rankings for apps. It's almost impossible for a small app developer to get a base amount of reviews, so they usually rely on friends and family to download and rate the app.
So, do you see what Google is doing here? Basically Google is strongarming developers to tell customers, friends, family, etc. to go get a G+ account to rate our app to help us out.
Promoting Google Plus at the developer's expense. A god damn shame.