This annotation has been the subject of some heated a.f.p. discussion (and if you think that this is a silly thing to get worked up over, you are obviously not familiar with alt.fan.pratchett. Or with Usenet, for that matter). Anyway, there were a few people who felt that Terry was referring here to Larry Niven’s Ringworld series, where the main character, Louis Wu, always uses the phrase “There ain’t no justice” (abbreviated as “TANJ”). Other people found this connection incredibly far-fetched for such a generic sentence, and said so rather forcefully.
Eventually, Terry stepped in and short-circuited the entire discussion by writing: “Mostly in the Discworld books, particularly Mort, the phrase is ”There’s no justice” so that it can be balanced with “There’s just me/you/us”. And that phrase is truly generic. Really, so is “There ain’t no justice” — it’s just that Niven does use it a lot and, I suspect, uses it because it is familiar to readers. Admittedly, it’s become ‘his’ via repetition. But there’s a difference between using an established phrase which another author has commandeered and using one specifically associated with one person — “Make my day” has one owner, whereas “There ain’t no justice” is a cliché. To be honest, I didn’t have anything particularly in mind when Charley uttered the phrase — but if you think it’s a Niven reference, fair enough.”