DiapDealer, I hadn't so much ignored your earlier post as decided it was possibly not appropriate to continue this on this thread. But since you and vrf seem interested...
The Golden Rule was cited as an answer to the question - it's why I see the behaviour cited as being wrong. And while the Golden rule may not ever have been
the social default, it has always been
a social default. (Think of the standard parenting phrase: "How would you like it if you brother/sister/someone did that to you?")
vrf, you ask why we should care about this rule, and the answer is in that link: because it is built in. This is partly evidenced by, as Wikipedia noted "The concept occurs in some form in nearly every religion and ethical tradition." Also...
The Golden Rule, as noted in the Wikipedia link I gave above, in evolutionary terms is called "reciprocal altruism". Understand this background you can also understand why people tend to limit their empathy - and so their application of the Golden Rule - to people they know, or people they see as being the same as themselves. But times they are a changing. Many things we used to reserve only for our immediate tribe we now extend far beyond. Not consistently, and not universally, but still significantly more broadly than our ancestors. We are now much more inclined (when we bother stop to think about it) to accept that the person on the receiving end may be someone not unlike ourselves, and to adjust our behaviour accordingly.
This subject is (slightly) more on topic that it at first appears. The Golden Rules is about empathy, about being able to see others as similar to ourselves and to be able to put ourselves in their position and see the impact of our behaviour from their perspective. Many of Pratchett's books included a theme that can be related to the Golden Rule:
From: Racism was not a problem on the Discworld, because -- what with trolls and dwarfs and so on -- speciesism was more interesting. Black and white lived in perfect harmony and ganged up on green.
To Granny Weatherwax's neat summary: "And sin, young man, is when you treat people as things."
In several of the books we see the various species (generically "people" in the eyes of the story) being brought together to understand that they were not so different after all, and so to understand that doing nasty things to the other species was like doing nasty things to themselves (often a literal truth, since the never ending cycle of retribution tends to produce such results).
He also refers to the Golden Rule directly in this
conversation with The Guardian.
So I do tend to assume that any reader of Pratchett is familiar with at least the concepts surrounding the Golden Rule, even if they do not necessarily recognise it by that name.
And who knows, real people may one day be as smart as some of the Discworld species and come to the same conclusion.