You're reading selectively again smartypants
Jason's saying the same thing as you but for a different reason, not the reason you're postulating - that there was no
time to get separate proofs for the US market.
Didn't we have this discussion about chapters GP and MM before? Terry's on record as saying that he liked the Dickensian method of giving little teasers in the chapter headings and GP certainly (haven't got around to MM still

) is very Dickensian in flavour. So Jason's most likely correct in this. Besides which it would cost money to make what are essentially quite drastic changes to the typesetting of the novel to add chapters - so it's just stupid to take out something that's been put in by by the author.
Terry's at the height of his craft and so he's playing with presentation and deliberately styling it meet the ambience he's wanting people to assign to these particular books. In other words
he wants to have chapters and teasers and because he's a hugely successful and respected writer
his publisher is following
his instincts and going along with it. There's no question of having a different publishing criteria for the UK and the US and even if there was and, as I suspect what you're saying here is that the US, being a highly conservative market, is dictating publishing styles in the UK, then why would they suddenly want chapters in Discworld books when all the others haven't had them?
US publisher: What in hell is Terry at here? Chapters?! He's never had chapters before! Why's he want chapters now? People won't like him having chapters! They'll hate it in fact! No! No! No!
Terry: I want chapters - they fit in with the tone of my book and I fancy a change.
UK Publisher: Hmmm - you have a point
*and we want to keep your account 'cos they sell like hotcakes* OK! We'll go for it if that's what you really want.
US Publisher (to UK publisher): What? You will?
*looks at calculator and sees some advantages in having chapter headings...* OK - if it's what you really
really want... When d'ya wanna to roll the presses guys?
If Terry didn't put the chapter breaks in himself then nobody else would have bothered. Simple as.