If you were punished by the instant feedback system, you should have been sent an email. If not, you can contact player support to get the chat logs specific to your ban.
And yes, repeatedly berating allies for doing things you don't agree with is considered harassment and will get you punished if you do it enough. There are ways to make suggestions to people without calling them scrubs. And doing that is not going to get you punished. Simply saying "Ahri, you should have let me finish that kill off so you'd have lived" would communicate the same thing in a much better manner.
Would it have changed Ahri's reaction? Impossible to tell. Since that response wouldn't be calling her out as bad, it might have been better received. Or maybe she's just a dick and doesn't want anybody telling her what to do so she feeds anyhow. But if she chooses to do that, then it's 100% on her, and you're clean. Responding as you did makes you as guilty as she is (had you said nothing at all, she probably wouldn't have flipped out and started feeding).
And again, I stress that the restriction was not due to simply this game if your behavior is consistent with what you're admitting to. It takes a lot more than a single game of calling somebody a scrub to get punished.
And you're right, feeders are a problem that isn't taken lightly. That is, intentional feeding. Many people mistakenly think the 'intentional feeding' report is simply for players who died a lot, which isn't the case. There has to be actual intent for it to be intentional feeding. Fortunately, they have a system in place now that automatically detects and punishes players who are truly intentionally feeding. So no, they're not getting away with it. And you don't need to jump on a vigilante justice campaign to 'set them straight'. Just report them after and they will get punished if they make a habit of it.