According to Reuters, Cook said Apple based the decision on "credible information" from Hong Kong police and Apple users. Those sources said the app was used to "maliciously to target individual officers for violence and to victimize individuals and property where no police are present." That kind of behavior would violate App Store guidelines prohibiting personal harm.
Critics argue that the app does not show the location of individual officers, so it could not be used to target law enforcement as Cook described. They say there's no evidence that the app has been used to threaten police or public safety and that apps like Waze, which crowdsources information about police locations are still in the App store. So, while Cook was likely hoping to set the record straight and quell the controversy, plenty of people are still upset with the decision.