Why don't we at least get a key from playing free champs that we don't own?

Just curious why they did this. I understand that RIOT don't have a whole lot of common sense but if we have to play a free champ because of a random pick , such as ARURF or ARAM we at least should have a chance at something for our time. I understand that we won't get a chest but at least give us a fragment if we do well. Either that or stop giving people the free rotation if they have so many champs. It's stupid. There are reasons that certain people didn't buy those champs. Use some common sense RIOT.