If you go read through US military documents about the use of force and rules of engagement you will find things like it requires _presidential approval_ to use riot control agents like pepper spray and tear gas. You aren't allowed to use non-lethal weapons like rubber bullets and bean bag guns without special training because they are dangerous if use improperly. It's against the law of war to target noncombatants (police have been specifically targeting journalists in a number of high profile incidents)
[0] https://www.trngcmd.marines.mil/Portals/207/Docs/TBS/B130936...
edit: I agree with the sister comment that the faulty police mentality largely comes from really bad training that instills the mind set that there is danger behind every corner.