However I honestly think it also doesn't occur to them they have a choice. Charles and William are not clever, had very emotionally stunted upbringings and were trained from birth that this is their duty. I doubt they are riven by internal debate on the ethics of it all.
This, basically. Never underestimate people's ability to mix up "is" and "should".
Comparatively trivial example: I grew up in a religiously quite mixed place, and I grew up knowing that "we" were protestants and that catholics were "different", a little weird and somehow wrong. Also: that we envied them because they had a lot more religious holidays.
Here's the thing: not a single adult in my family was, in fact, a believing (never mind practicing) member of any church under the rather large umbrella of "protestantism". People's actual beliefs ranged from "will get married in church" to "won't even have the kids baptised - they get to pick when they're grown up". The latter, in fact, were my own parents - so despite "us" being "protestant" and it figuring in our identities, I personally, technically, never even was.
I genuinely didn't figure this one out until I was in my teens. And comparative speaking, it was a very inconsequential insight: neither my own future trajectory nor that of my family or future children was at stake. The worst consequence I "suffered" from realising I wasn't nearly as "obviously protestant" as I had grown up to believe were a few days of intense head-scratching followed by the acceptance that humans will human.
Point being: I don't think we should presume deep thought when "just convention" is a reasonable explanation.