Thanks for the 10 bans Riot

Never hover over and let them see you want to play him.

It may have been intended to let everyone feel like they have the ability to ban that one champion they just really hate laning against. Most players, from the most casual even up to the professional players, have champions that they know they do not want to face. Sometimes it's a champion they cannot beat in lane, sometimes it's a very strong jungler, or the flavor of the month that seems too strong. It gives everyone a chance to set the game up a bit more in their favor right now.

There will always be some trolls and griefers who like to ban their team's picks. But they are not the majority of League players, and it should not happen too frequently. If you do have someone who is doing this deliberately, report them for it in the post-game lobby. There is a report category that covers having a negative attitude that is not directly tied to harassment or verbal abuse/hate speech. I think it's the first one. Report them, and under the additional comments, type that they were deliberately banning teammate's champion.