Our sorcerer got some officers to confront the person who stole his identity and we turned the human trafficking ring over to the guard who pulled everyone out relatively safely.
But to even things out the sorcerer killed someone for his cloak and my rogue stole stuff from a person we later found out leaves the thieves' guild the next session. Karmic balance.
I mean we can basically justify the law keeping faction to act and respond in whatever way suits the story.
Perhaps the police are a mercenary company paid to guard the local clergy, so reporting their misdeeds is moot.
Perhaps as you said it's that the law keepers are an order of paladin's who must uphold their oath of enforcing the law or lose their magical connection.
Perhaps the closest to a guard force up to that point has been the ruling warlord's loyal warriors, and to your party, they're the enemy.
If the party become too over reliant or too wary of the police, have them reform in plot, for better or for worse.