That's because there's a deleted scene where they're drunkenly singing it after they leave the 3rd class dance.
The dead woman's family feel that her ex-boyfriend is to blame but Frost believes the priest is more involved than he is letting on.
A final scene shows Jack saying goodbye to George's coffin, and telling Christine that he is ready to live a life outside of his vocation of being a policeman.
Perhaps a remake is in order.
The character had appeared in a movie called Born Losers back in '68 but didn't cause any waves until the 1973 re-release of Billy Jack it was first released for a limited run in 1971.