This is so bazar that I don't even know where to start to try an understand it.
Is it a joke? Tongue-n-cheek? It must be. I can't understand how it could possibly be a ligament concern. What about other languages? As adults who doesn't use 'fuck' conversationally? If it were a real concern then surly drawing attention to it in the design of the ID is a more obvious then the very rare occasion of 'dirty words' (by some definition of dirty) appearing in otherwise random strings.
So I have to conclude it's a joke, but then what the fuck?
This type of thing puts noise into engineering details that just ends adding to confusion and ambiguity. If there's no (important) reason for 'U' to be in the omission set, then just say it's an arbitrary choice, if there is an OCR, or legibility reason then say that.
Half of my brain says it's funny, and half my brain says it's fucked up.