ARAM and key fragments.

So, i just did an amazing job on my first time Karma in an ARAM, i received an S+, BUT because i dont own the champion i dont receive a key fragment or chest? I feel thats unfair, i understand if a teammate gets the grade and you dont own the champion, but i did the best in the game, and received the highest grade, yet, still earn nothing. Why even give the option in the first place in ARAM if there is now 20 free to plays, obviously i wont own everyone yet.