According to
IOL Prince William and Prince Harry have asked Bryan Adams not to appear at the concert honoring their mother. The Princes did not want the gossip of Adam's secret fling with Princess Diana to overshadow the event. "The princes have nothing against Bryan Adams… But they want the concert to be free of any innuendo or tittle-tattle. It was decided that it would be inappropriate for Adams to appear." Adams confirmed that he will not be playing the event.
I can understand why the Princes banned him from doing the show, it should be about Princess Diana only on that day.