Sorry, you said it's random but that means it happened before and you know you're prone to have such d/cs already. Which means you can't afford to intentionally afk.
OK you don't leave games once a month. You tilt and leave 'quite often', and stress the fact that you did not leave a game in a month now, which sounds like a big record for you. To me it means you usually leave the game more often than that. AKA more than once a month.
I don't agree with the system, I complain about afkers a lot, but I try to obey the rules, and so should you. Perhaps learn how to deal with tilt, I know I need to. But I don't just leave my team, because I've been afked on and it sux. We have a saying, 'what you do not like, do not do to others'. In English being 'do as you would be done by'.
If someone does go afk before you do, imagine them reconnecting (it happens, perhaps they put out that microwave fire) to find that YOU're not in the game to make the comeback real.
You're just adding to an already big issue.
Glad to hear you accepted the punishment, and please don't try to ruin the game for other people.