Is there a rhetoric of italics?
What does it mean when we italicize something? The most likely account is that using italics calls attention to the choice of word. In the first place, it asserts that there is a choice, and that the author deliberately chose the word or phrase, as opposed to others. The word choice might depends on some aspect or shade of meaning compared to a synonym or a more or less general term.
Oddly enough, the sentence above seems to me to be a slightly different case. Here, the word "is" was italicized, and, while one might say it was to foreground the choice between "is" and "is not", it seems to me to be highlighting the existential "is", to make it more prominent, and that this is needed because normally the verb "to be" isn't as important as other words, so if you want to talk about mere existance, it helps to call attention to it.
On the other hand, sometimes italics is just a way of calling attention to the term, as one that will be used again, or that has a a special sense in the current work. (Boldface or small caps can be used this way, too.)
In this case, there is certainly a discourse function but perhaps not a rhetorical one. That is, it's important for comprehension, but not for the purposes of persuasion. (I take for granted that comprehendability (intelligibility) is logically prior to persuasiveness. If you can't understand something, it can't possibly persuade you. And you can understand texts without being convinced by them, and for that matter, not all texts are intended to be persuasive).
When italics serves to mark a new term (and using an old term in a new way counts as a new term) it can't possibly be persuasive itself. Things can only be persuasive by being linked to things the reader already accepts, so introducing a truly new term can't get you anywhere. So what is the italics doing? Well perhaps we can explain it using Grosz and Sidner's framework, where there is both an intentional (roughly, rhetoric) structure and an attentional structure, which is a stack. Perhaps italics serve to mark the term as one particularly worth paying attention to, warning the reader that it will be used again.
But it might also convey something else.
Now as for using italics to indicate that a choice was made, I think it's likely that classical rhetoriticians had a name for such devices, especially that of using a more general term than necessary. (Consider this dialog:
Finally, you might be able to give a Gricean account of this device. Using a too-general term is perhaps a violation of QUANTITY, because the speaker has used a longer description than needed ("him" would suffice).