My guess is it helps people navigate the fear.

So either the cops are saintly and don't do anything wrong, or they're all evil demons on a neverending quest to murder as many black bodies as possible. And if you don't believe the latter, you MUST believe the former.

Having just two choices makes everything alot easier to handle.

Of course the truth is in between those two, but then that makes coping with danger much harder, because you have to think more and use reason more.

And when people come along who point this out, they have to be dismissed or ridiculed - made to be quiet and/or go away, because we're disturbing the holy/evil dichotomy.