The decision was "not easy," Cook said in the email, adding that "technology can be used for good or for ill."
"The app in question allowed for the crowdsourced reporting and mapping of police checkpoints, protest hotspots, and other information. On its own, this information is benign," Cook said.
But then, Cook said, Apple received information from users in Hong Kong and from the Hong Kong Cybersecurity and Technology Crime Bureau (CSTCB) that the app was being used to target individual officers and "victimize individuals and property where no police are present."
"This use put the app in violation of Hong Kong law. Similarly, widespread abuse clearly violates our App Store guidelines barring personal harm," Cook said.
In a series of tweets, HKmap.live said it disagreed its app put lives in danger. All the app does is consolidate info from news streams, Facebook and Telegram, it added.
"There is 0 evidence to support CSTCB's accusation that HKmap App has been used to target and ambush police, threaten public safety, and criminals have used it to victimize residents in areas where they know there is no law enforcement," it tweeted.
It then accused Apple, the NBA and retailer Tiffany, of suppressing human rights and freedoms.,
Google has also reportedly removed a mobile game, called "The Revolution of Our Times," from the Play Store that let players role-play as protesters in Hong Kong. The Hong Kong police requested that Google remove the app, according to The Wall Street Journal.
The Hong Kong protests, which started in March, initially focused on a bill that would have allowed people arrested in Hong Kong to be extradited to mainland China. The bill has since been withdrawn, but demonstrations have expanded in the past four months to include other grievances and demands for greater democracy.
Blizzard has also faced backlash over the protests. It removed pro Hearthstone player Chung "Blitzchung" Ng Wai from a tournament Tuesday after he made a statement in support of the demonstrations during a "Grandmasters" competition. He's "ineligible to participate in Hearthstone esports for 12 months, Blizzard said.