Haven't really answered the "why" question as far as I can see.  I am a retired Forensic Specialist with a Canadian law enforcement agency.  I was a street grunt cop for my first 22 or so years.  An eye  in the sky is a valuable asset for many operations that the police undertake, and it is a lot more economical than leasing a helicopter.  Missing persons, crime scene documentation, swat team overview, traffic collision/fatality investigation.  I'm pretty sure that even in the States like Canada, Police have to apply for permission to enter restricted airspace to carry out their operations; where they'll be, how high they'll be operating and for how long.