The reports of abuse came out before the first game. He was only formally indicted after the first game. It wasn't until the Ray Rice video came out, though, that the fit hit the shan. Before that, no one even contemplated Peterson sitting out a full season.
But does it really matter to the point of this discussion? If a team has the choice of (1) having a large salary count against their cap without the player playing all season and a potentially large portion of the next, and (2) simply cutting the player, then it's obvious teams are going to choose the second option in many cases. Thus, the player isn't given much of an option in whether to agree to the exempt list or not.
It's a severe form of punishment prior to actually being punished. That makes zero sense to me.