It’s a slogan used as a protest statement, particularly within anti-establishment and anti-police movements. The acronym has a long history, particularly within subcultures like punk, skinheads, and football hooligans. It’s also associated with anarchist and anti-authoritarian ideologies.
Then why do Americans constantly say “all cops are bastards?”
It also originated in England.
America is not nearly based enough for Americans to “constantly say” that, unfortunately.