For a long time, Apple has banned every and all apps related to one controversial issue: medical marijuana. It's all there in the App Store Guidelines:
Apps that encourage excessive consumption of alcohol or illegal substances, or encourage minors to consume alcohol or smoke cigarettes, will be rejected.
The app MassRoots for both Apple and Android is among the banned apps because it connects medical marijuana users to dispensaries where they can purchase their medicine, to doctors and caregivers, as well as other smokers and users. So, the problem with guideline 2.18 above is the word "illegal."
Because the use of medical marijuana is legal in 23 states (with three more states with pending legislation) and recreational use is now legal in Washington and Colorado, the app's makers appealed for MassRoots to be allowed back into the app store.