The Mishnah in Berachothch2? teaches that one may recite Shema' until the end of the third hour of the day, "for it is the way of the sons of kings to rise by three hours."
Why is that the deciding criterion? Is the Mishnah saying something profound about Bnei Yisrael being the sons of the King of kings, and that therefore the usual practices thereof ought to determine what time we recite Shema' (if so, why)?
Or is it saying that the sons of kings are the latest to rise, and so whatever time is normal for them to sleep in is the latest acceptable time for us to recite Shema' (if so, why)?
If either of the above is correct, why would the time be permanently fixed based on some practice of sons of kings eons ago?
Or is it something else?