A_Slab_of_Bacon beat me to it. The 2020 season has been an asterisk season from the start. There are many ways that the Big Ten could have handled this better. One additional way would have been to flatly state that with all the craziness of covid and the muffed start, they reserved the right to make future changes as necessary to deal with unforeseen conditions. That would have at least put the schools and fans on notice.
That said, this should have been a foreseen condition. Most of the schools were concerned about covid, they put strict 21-day rules in place, and therefore should have realized that there was a high likelihood that a top team would not reach six games. If they did foresee this and made the 6-game rule....then they should have stuck with it.
I wonder if they would have exempted Indiana had they beaten Ohio State but were also short of six games.