Comparing it to Facebook or Snapchat makes absolutely no sense.
Considering the brutality of the HK police force in recent months, I would argue that this app is great at providing the public with information on how to avoid being tear gassed, beaten or arrested by avoiding the police. A lot of locals have been extremely pissed off with the police force. They have gone into very residential areas and fired tear gas unnecessarily among a whole host of other things.
Since when is that a crime?
When news reporters cover a protest in US and show live where the police is, are they committing a crime?
Incidentally if he posted about a riot in our city (thankfully pretty much unheard of) he'd be carrying out exactly the same function as this app!
Following US procedures in HK does not make sense. Companies always obey local laws but not laws in another country.
After that, the next question is if Apple, which even kinda positions itself as the new "Don't be evil" company including fighting some governments e.g. when they get told to subvert their own security tech, should side with such draconian laws or with the app makers.
The vast majority of people do not post or read those messages to coordinate violence against the police, but merely to stay out of the way when police and black block protestors or football hooligans clash.
I live near a football stadium, and there are some violent clashes of drunk hooligans and police after many games, but thankfully usually only on a small scale involving maybe 10-30 people against the police. Nonetheless, I made it a habit to avoid certain area for a couple of hours after a home game.
Perhaps avoiding police is the primary use case of the app.
But those alleged crimes could not have possibly helped by the app, as has been pointed out: you don't get to see individual officers' locations and it does not show areas with no police at all either, just police hotspots (and technically, most areas even in a dense city like Hong Kong are without immediate police presence most of the time, anyway).
But even if we played devil's advocate and took the allegations of criminal activity that abused the app at face value, and assumed Tim Cook is not free to share specifics as the information might be confidential, he could at least answer what local laws were allegedly violated by the app itself. Those laws certainly are not confidential information.