According to Touch Arcade, Apple has made a change to this system. Starting last week, App Store users cannot review apps that have been downloaded using a promo code.
Apple hasn’t released official details — or reasons — for this change. More than likely, this change is designed to help curb the positive effect of promo codes on reviews.
The lead developer of Promo Dispenser contacted Apple Support who confirmed this change. An AppleCare agent told the developer:
Anand here again from iTunes Store Customer Support. Thanks for writing back and letting me know your concern.
I understand that you are still not able to write a review. I know how disappointing it can be when things don’t work out the way they should.
I am sorry to inform that it is no longer possible to rate or review an app if it was downloaded using a developer’s promotional code.
Article Via Mac Rumors