Pink you’ve made a number of assumptions in drumming up your outrage:
You’re assuming the William was given a choice about the date.
“power” what power does William have as third in line to the throne?
You’re assuming that he shrugged his shoulders and said “who cares about the football”. For all you know it was a very hard decision and he feels bad about it.
Something would probably always have had to be bumped, would you prefer it was a charity or diplomatic event?
It’s not like William is some playboy who runs around drinking and gambling. He has a number of commitments to honour.
He seems pretty committed to supporting sport in the U.K. perhaps ge’s Just being a good brother and sucking up his disappointment about the cup final at the request of his Grandmother and Father.
I’m a Republican and even I think this isn’t unreasonable.