I'm really wishing we can get someone with authority to give us a word so we can continue judging cases with confidence. But until then, all we can do is continue giving our points of view, spur discussion, and understand each others' perspectives and be able to come to a fairer conclusion for ourselves.
I agree that being banned for leaving a couple of bot game is pretty ridiculous. But at the same time, I don't think that it's okay to leave any game, be it a bot game or not. Of course there will be situation when something urgent happens and we simply cannot wait for the game to end, but such situations should be few and far between... and if someone were to make it onto a tribunal case for leaving, I would like to be able to assume that such behavior is a pattern, and such patterns should be discouraged from continuing.
The tribunal is the system set in place to help enforce the summoner's code, and leaving for any reason violates a few tenets of the Summoner's Code:
I - Support your team
"Also, remember that you have to be there to contribute, so don’t leave the game or go AFK!"
I guess this is as close to an answer as it gets. It doesn't say anything about bots... so I'll asume that any game mode where you have a team, it is your responsibility to support it, no matter what kind of opponents you are facing.
IV - Enjoy yourself, but not at anyone's expense
Leaving a game means you are ruining the game for at least 4 other people.
IX - Lead by example
"Nobody’s asking you to be perfect, but we do want you to, whenever possible, strive to uphold the same standards of behavior that you expect everyone else to maintain."
I don't want people leaving during my games, so I should be expected not to leave games myself. Makes enough sense to me...