If you swear in San Angeles (or so the movie tells us) you get fined for doing so. The slip up here is that most times, Sylvester Stallone lets fly a four letter word and nothing happens; i.e no slip of paper issues from a machine either in a building or in a patrol car.
Technically, yes it's a slip up that I've thought about when I used to watch this movie all the time. But I think the joke would become obnoxious every single time Wesley Snipes, or Stallone let a four letter word fly and you hear the buzz. I think the director cleverly let the joke die after Snipes original run in with the S.A. Police and Stallone's need for TP