Eh? A bunch of random farmyard animals are all equal?
It's all got to do with the police. While in Germany, policemen are called "die Bullen" ("the bulls"), in Britain, they are "pigs" and in France "chickens" ("les poulets"). Personally, I think it doesn't sound too bad to call someone a "bull". After all, they're hefty animals that can crush you if they want to. That's the kind of policeman you want, someone who's nice and burly (stämmig, kräftig), someone who can deal with criminals resisting arrest. "Pigs" on the other hand - just not nice at all. Although pigs are said to be very intelligent animals, that aspect of their personality is not what one thinks of when conjuring up an image of a pig.
"Chickens", on the other hand, why? Well, apparently, it goes back to the latter half of the 19th century. The police headquarters in Paris was burnt down by the Communards in May 1871. They were rehoused in some barracks that had been built on the former site of a poultry market. Hence the name "les poulets" or "the chickens". Much cuter than "pigs".
