The only scenario that makes sense to me:
Sharpe didn't think he was eligible, wanted to get a head start on next season. Got on campus, found out he was eligible, his inner circle advised him not to play once mock drafts came out that showed he was in the lottery, in some cases top half of the lottery.
At that point Cal couldn't really do anything that the fans would have liked.
Can't make a guy play who doesn't want to.
Cal already told him he could prep for next year w/o playing. Would look badly on a coach to renege on something like that.
Kicking him off the team because of refusal to play looks even worse.
So you accept the only positive thing that can come from it, a slight association w/ a possible NBA star.
Yeah, sucks, I don't expect him back either, but I can't come up with a reasonable scenario where Cal wanted it to play out like this. I think most people who aren't pitchforks and torches can see this reasonable angle.
If it comes out that Sharpe knew all along he was eligible and knew he'd never play, well that's a different story, but we have no evidence of that.