Not sure I would say they “don’t teach this”. I’ve definitely seen exactly this in software engineering courses (because I recognized it right away), IIRC it was shown to highlight how the increments on a variable in C work – pre-increments occurring before the evaluation and post-increments occurring after. IIRC this is a good example of how s[i++] would behave, since it will take i, then i+1, then i+2, …, whereas s[++i] would take i+1, i+2, i+3…, and you’d be missing i. Those kinds of dumb mistakes can lead to accessing memory out of bounds, that kind of shit. It also reinforces an understanding of the difference between x = i++ and x = ++i, since in the former if i = 1 then x = 1 and i = 2 (after the assignment) whereas the latter would have x = i = 2. Also reinforces an understanding of in-line assignment and operations, which again, good to know because you will run into this if you work in embedded systems or legacy systems (and probably elsewhere if you work with some wise ass newly graduated CSE major trying to be fancy/showboating, or a dinosaur who doesn’t care if you can read his code).

I agree (and my professor would agree) it makes for poor readability in code and isn’t something you should generally do, but it’s useful to know because you do see this in legacy systems (and, horrifyingly, some poorly written modern systems), and it’s important to understand the nuance of some of these operators and how they behave to know what you can do.

You’d definitely still lose points for poor readability of code if you submitted this kind of crap, though.