This argument goes around and around every time someone says ACAB. There is a culture of protecting your own in police forces - it seems to happen all around the world. This culture causes otherwise good cops to overlook actions of their peers that shouldn't be overlooked. If you let someone get away with something illegal, when your job is to uphold the law, then you are not doing your job and are really not a good cop after all - you're just as bad as your peers. ACAB is a much simpler way of saying all that.