Some departments are just more open about it than others.
As you say, I think a lot of cops do get into it for basically-good reasons, but the (hopefully-small) number who're in it for the power trip or who become dirty over time are fundamentally corruptive to the organization when combined with the "thin blue line" sentiment. If you can't trust any cop to help when you're victimized by another cop, "a few bad apples" will have spoiled the bunch.
Of course, I'll admit that much like "defund the police", it's kind of an umbrella term that gathers a few different views together, and you can't tell whether the person saying it holds the view I expressed above (which I think is the most mainstream position), or whether they think that policing is inherently corruptive and should be removed from society somehow.
First off: this is not about America. Read the title, and stop being self centered for a moment. If you think France hasn't been saying ACAB for decades now, you're sorely mistaken. You know what my experiences are with the police, as a regular white dude ? Not what I see on TV, what I have lived. It's them not intervening when a neighbour in the middle of a psychotic episode stabs a passerby. It's them refusing to take in a deposition for sexual assault. It's them checking my arabic friends for drugs, but leaving me alone when I'm hanging out with them.
So, if you want more nuance: ACAB, and defund _this_ police. A few bad apples spoil the whole bunch, well the basket is rotting at this point.
Citation needed.
The slogan may be counterproductive to achieving its stated aim, but it's also opening the ol' Overton window to a range of possibilities beyond the usual vague but well-funded "reform". If that's what it takes to get a marketplace of ideas, I'd say it's productive.
Slogans like #ACAB and "defund the police" shut down debate on reform. They're most successful at consolidating the pro-police position with moderates by presenting the entire police-reform spectrum as anti-cop radicals.
It's a bad guess.
EDIT: Police in America have a "thin blue line" problem of reflexively defending peers' mistakes. Their cultures look disturbingly similar to gangs, though I don't think that's intentional--they're better described as fraternities with guns. (Which also describes gangs. Convergence.)
That said, new recruits aren't corrupt. And most cops are trying to do the right thing. More fundamentally: most Americans want a police force, and when the police stop working, the most proximate token is torched.
So yes, I get the drive behind the slogan. But it's counterproductive to a ridiculous degree. And it's nowhere proximate to a solution. On HN, I figure we can do better.