Private institutions still have rules to abide by due to their interaction both with licensing and finical aid. For example BYU has to abide by Title 9 polices.
Private institutions in this day and age have to give over some of their control for their degrees to mean anything and for the students to compete in the general population.
Been there done that from the institutional side.
When I graduated, I had to wait about a month to receive my diploma due to some verification of my internship they needed. During that time, I was still officially a BYU student as was this guy. Had I violated the Honor Code during that time, they could have done to me the exact same thing they did to this guy. Why? Because I was still a student and therefore subject to the agreement I signed. The agreement ends at graduation.
Had he waited until they handed him his diploma and THEN published his calendar, BYU could have done nothing to him. So, he could have avoided all of this but he chose to break the Code while he was still subject to it. And given the timing of this controversy coming in the wake of Proposition 8 and his own statements to the media I posted above, it would certainly seem he deliberately chose to create this situation.