Pre-Selecting a champion in champion select should automatically lock you in as that champion if afk
If the person has selected a champion, they are very clearly there and intent on playing the game. Having to lock in manually and getting kicked if you don't seems a bit silly to me. If someone hasn't selected a champion then yes, just kick them obviously.
I personally only bring this up because of a connection error that I had earlier. I had selected my champion, but when it came to lock in, my chat disconnected and I wasn't able to, even after reconnecting a few moments later. Having it auto-lock your champ if you don't lock it is a nice fail safe for this, and I can't really see it being abused. (Though please inform me if it can, I would like to know)
Also if this has been discussed before, my bad, but it's also nice to get updated responses.