Why Can't I Rate an Apple TV App?

I've seen a response that there should be a "SELECT TO RATE" button in the Ratings section but I don't see it. The app I'm trying to provide a rating for currently has 1.9 so I know others have done it successfully. The app that I'm trying to rate is "Cloud Player" by "luo nao". You can't actually buy it but it has in-app purchases to get access to dropbox. When I tried to view photos it had 2 problems that make it unusable: 1) as soon as you view photo there's no way that I've found to go back to the file list and you cannot navigate to previous or next photos- I just have to kill the app; 2) portrait photos are stretched (actually, probably all photos are stretch but portrait photos look horrific). The app subscription was $1.99 for a year so it isn't that much money, but the app is actually useless to me so I'll be getting the money back.