If you care about forum avatars, then you spend too much time on the forum compared to how much time you spend on playing the game..
1# Forum avatars could easily be abused by adding sexual content that would take time for riot to discover before getting put down.
2# if would require time from employees / more employees to check up on avatars and see if any rules are broken that would have anything to do with racism, sexism etc.
3# It would require more data.
So... why bother? It's not like it's going to enhance your game experience, and riot is a game company, just fyi in case you should've forgotten.