Some people here would probably call that a "dark pattern" (I of course don't). Because, you know, it's soliciting for positive reviews from a preselected set of users.
I don't know why people write AppStore reviews though. If I like an app, I'll go on my computer and tell people via social networking. Typing out a credible-sounding review on my phone sounds like work, not fun.
> Using the SKStoreReviewController API, you can ask users to rate or review your app while they're using it, without sending them to the App Store.
[1]: https://twitter.com/iCalvin/status/823977790382993409/photo/...