I don't recognise the Pratchet reference here but I vaguely remember a number of obvious influences striking me at the time - unfortunately I was too ill for reviewing at the time I read it so details are escaping me...
The whole "organized guilds being permitted or even tacitly encouraged to commit a certain number of offenses that are then not prosecuted thing -- as 'the devil we know' -- is the Pratchett reference. Only Pratchett uses is satirically, both vis-à-vis society and the system of law and order, and vis-à-vis organizations such as the mafia. Here, there is no satire in play whatsoever.
And yeah, this is definitely not the only element of the book copied from elsewhere. It's just the one that's easiest to pinpoint in a single dialogue ... (and to me personally, the most annoying one. So far.)
Can't remember where it occurs for the first time, but it's definitely in "Wyrd Sisters" (which I just reread for the umpeenth time). If you never progressed as far as the Night Guard books, though, it may not have stood out to you in the earlier books. But you can't miss it (even in the earlier books), once you've read at least one book of the Night Guard subseries.
... all of which (well, except for "Sourcery") really tells you nothing about Discworld. Seriously, if you've given up on that series before it really got going (and that was with "Wyrd Sisters" -- the book right after "Sourcery"), you have some major catching up to do, because you've seen comparatively little -- almost nothing -- of what makes the series so great.
And yeah, this is definitely not the only element of the book copied from elsewhere. It's just the one that's easiest to pinpoint in a single dialogue ... (and to me personally, the most annoying one. So far.)