They don't consider this as a modelling assumption, but rather something that is actually true.
Of course, in cultural evolution, mutations are not random. Instead they are what Donald Campbell (confusingly) referred to as "blind" - which he defined to mean: "based on existing knowledge". This just means that they are nor prescient or miraculous - but rather are naturalistic.
Many have claimed that this makes cultural evolution different from organic evolution - where mutations really are random. However, mutations aren't really "random" in the organic realm either.
Unfortunately, the idea of "random mutations" has turned into a morphing concept. It represents goalposts that shift - whenever the idea that "mutations occur at random" is falsified.
Consider how Jerry Coyne defined what biologists mean by mutations being "random" in his book "Why Evolution Is True". Jerry said:
The term “random” here has a specific meaning that is often misunderstood, even by biologists. What this means is that mutations occur regardless of whether they would be useful to the individual.This is what I learned the term "random mutations" meant (in evolutionary biology) when I was growing up.
However, now Jerry is saying something very different. He now says this:
What we mean by random mutations is simply this:
“The chance of a single mutation being “adaptive” for the organism (i.e., promoting the replication of the gene in which it occurs) does not depend on the environment in which it finds itself.”
See how the goalposts have shifted here? In the first quote, there's no mention of dependence on the environment. In the second quote we have this entirely new concept.
Of course the reason for this is that the first statement is now widely recognized to be false. Mutations are just not random in that sense. The most harmful mutations are the ones most likely to be the target of error correction mechanisms and it's widely recognized that (harmless) mutations in junk DNA are more common than (deleterious) mutations in coding regions.
Jerry's second statement sounds harder to disprove - but it's still wrong. Basically Jerry's assertion boils down to saying that the fitness of mutations is, on average, environment-independent. However, we know that is not true: mutations are more likely to be adaptive in some environments rather than others. Here's my comment, explaining why the new idea is still wrong:
The chance of a single mutation being adaptive for an organism does typically depend on the environment in which the organism finds itself. Some organisms are on fitness peaks - where the surrounding fitness landscape all lies below them and there are no nearby mutations that give a fitness boost. Some organisms are climbing "fitness ridges" - where there's some scope for going uphill and others are on "fitness slopes". In these latter cases, some nearby mutations do result in adaptive, uphill progress. The the chance of finding an adaptive mutation fairly trivially does depend on where you are on the fitness landscape. So: mutations are not typically 'random' - in the proposed technical sense given in this post.This is an example of a case where debating the dogmatic neo-Darwinians is so frustrating. You show that they're wrong - but then the goalposts get moved and everyone pretends that that was where they were all along.
In the case of the first definition, the term "random" was excusable. However, in the second case, using the concept of "randomness" has become pretty ridiculous. Jerry's post contains a whole section apologizing for the terminology. However, it isn't just the terminology that is bad here. The concept is wrong as well. Random mutations are a (very useful) modeling assumption. Mutations are not actually random - in any useful sense of the word.
Kevin Kelly is right about this - though for the wrong reasons.
Mutations in evolutionary theory must be constrained. Unconstrained mutations are consistent with any observations - and so explain nothing. One day, biologists will give credit to Donald Campbell for correctly characterizing the constraint on mutations in evolutionary theory. The history books will say that his discovery was not widely recognized among biologists for many decades - due to neo-Darwinian dogma.