Apple has been ardently working on improving its App Store ranking and review system as of late. Just recently we reported that Apple had tweaked its ranking algorithm in an effort to improve the legitimacy of App Store rankings.
Now another new policy to help support their efforts has apparently gone into effect. Any app that has been obtained by means of a developer promo code is no longer eligible for review.
In fact, Apple explicitly states that if you want to review any such app, you will need to actually purchase said app...