IMO ACAB is great on a behavioral science level: it sets the anchor/starts the conversation with a statement as an absolute, which the defense then has to find exceptions to. While they do that grunt work, the catchy phrase enables confirmation bias in the fence-sitters and people who already agreed with it.

It’s like starting a barter at an exorbitantly high price so the buyer is primed to offer a price higher than they would’ve before hearing the anchor.