I don't even think that it was a flagrant foul. It was a hard foul, but it didn't look like it was malicious. I just disagree with your premise that Lue shouldn't have tried getting the refs to take a look at it. When a guy gets hit in midair and the defender doesn't make a play on the ball I don't see anything wrong with wanting the refs to review it. That doesn't mean they have to call the flagrant but it gives them a chance to see if they missed anything in real time.
At home we can pause, rewind, all of that, but from where Lue was standing he sees his star going up for a layup and then someone knocking him down. Any coach is going to want the refs to review a play like that.