Some people whose opinions I respect said that this was a horrible slogan to use because it would be immediately seized upon by conservative troglodytes and used to discredit any attempt at reigning in police departments. Some said anyone using this slogan was stupid.
The people in my circle who dislike the slogan are broadly on the left. None of them voted for Trump in the past or will do so in the future. And they, and I, are all old enough to remember where a strong left stance on a particular issue was misrepresented by people on the right to the detriment of those on the left. So I think they are honestly worried about the same thing happening again.
I still wonder if the post-Reagan defensive crouch that many on my side of the political spectrum have been in may have blinded us to how politics and movements actually work.
The Right doesn't actually run on a slogan of "We're going to cut taxes for the wealthy and corporations, repeal or ignore any laws or regulations protecting the environment and humans from dangerous pollution, waste or chemicals, destroy unions so that the reserve pool of underpaid and unemployed workers remains high, shift public funds to private and religious schools, eviscerate legal protections against racial discrimination, eliminate free speech protections for critics of the Israeli West Bank occupation, hunt certain animals to extinction, eliminate worker safety protections, and drive down American wages via relentless outsourcing and automation so that corporate profits remain high!".