Again, you don't want to go down this rabbit hole. It is murky I admit and hard to follow.
Basically, the allowable reviewable plays state that you can review "advancement of a fair catch", which they interpret as both valid and invalid fair catches and obviously what happened as it doesn't become illegal until Cooper advances the ball.
However, it is a 5 yard penalty for "advancement of a fair catch", and the replay official simply called it Invalid Fair Catch and dead ball at the place of where Cooper fielded the punt.
Why would they do that and not give Iowa a 5 yard penalty as they should have?
Because "advancement of a fair catch" isn't allowed in the penalized play list that can be created in the review booth. The Big 10 office used the reasoning of "advancement of a fair catch" as the reasoning why the play was open to review, but then didn't use that in their conclusion because they can't go back to review and create a penalty for that.