Do we know that they did not give him a choice?That's his choice though. As it is, ICC didn't give him a choice.
His dis-appointment from Perth was not announced 15 minutes after the Sydney game ended or something. A good 3 days passed and the issue was big enough for there to be the possibility that someone went over and had a chat with him suggesting that he himself cop out. Bucknor himself also had opportunity to walk over to someone and suggest the same.
Not speculating that this happened, but the chances of something having happened are atleast as large as it not having happened. Or else, I am overestimating ICC's or/and Bucknor's sense of righteousness. Or perhaps the Darrell Hair episode, where he suggested a perfectly sensible solution of ICC settling his contract for $500K and ICC went public with it, has shut that door forever. (there was absolutely no need for ICC to discuss that with the whole world).