Thoughts. By the logical understandings, banning someone else's champion in your team inflicts the ethnics and the moral code that this game has installed.
We all have a right to our own thoughts, thinking, ideas, freedom of speech etc.. Intentionally not letting your teammate play the champion he wants to play creates the "conflict of rights", which is morally and ethically wrong.
After observing the rights every player should have in this game, i came to the conclusion that the option of enabling the banning of a champion your teammate wants to play in champion select is simply wrong.
If there is a possibility, i suggest Riot creates some sort of an automatic system which could detect the picked champions of your teammates and therefore prevent other players from your team from banning them.