Why no Cannons on Police Helicopters?

Wouldnt a cannon on a police helicopter solve a lot of
problems for law enforcement?

Is it possible to put a round right through the engine block
of a suspect fleeing and stop them from killing innocents?
Seems like that kind of accuracy is possible now isnt it?

When those guys were robbing the bank in LA, seems like
it would have been an easy shot to make from a helicopter
even without a cannon.

Whats up? public outcry or something?