S on champ I don't own, No chest for me but one for teammates?

After a recent game on Kindred, whom I did not own at the time, I received an S grade but was notified that I needed to own the champ to get the reward. Fine. What bothered me was that my friends later told me that they all got one because of my S. So, I'm essentially not getting rewarded for my efforts and good play, but everyone else gets a free pass for a chest? Why? Do i get it if I buy the champ in the future at least? I don't understand this design strategy.