I posted this as a comment on Thomas Basbøll's "Popular Notions," and it got so long that I decided to post it here as well. It probably doesn't make much sense without reference to Basbøll's post, which is here.
The linguistic concept of "register" is helpful here. People speak in different ways in different contexts. The register appropriate to a university course is different than the register appropriate to a television show, and both are different than the register appropriate to Twitter, or those appropriate for scholarly articles or for books like de Botton's. Even de Botton will write differently in his books, his newspaper commentaries, and his tweets.
These differently "appropriate" forms can be understood as "arbitrary" (and hence perhaps having more to do with power than with knowledge), but they can also be seen as having evolved to fit each context (and hence having less to do with power than with knowledge). The university lecture works the way it does not because the professors are asserting their power (or at least not only because of that), but because that form (with its appropriate register) has proven to be a useful way to perform the task of communicating knowledge.
In this sense, the types of critiques you're calling out, Thomas, are missing the point in more than just the way that you point out: without the idea of appropriate registers for different contexts, the varieties of language used in each context can easily seem like arbitrary expressions of power.
I work in Basel, where the everyday language is Basel German. But the University's courses are taught in High German (or in my case, in English, as I work in the English Department). The switch from "dialect" to "standard language" that takes place as the course starts seems arbitrary, but once it seen as a matter of register (or "code switching," if you prefer), then it is completely natural. The participants in the courses make the switch without any sense of dissonance at all.
The concept of register is also useful for thinking about American speech these days in general. One feature of contemporary American public life is the loss of the formal register. Al Gore's public "stiffness" was not actually "stiffness" at all; it was just that he used a formal register that is going out of style.