You weren't punished for misbehaving in one game. You were punished for misbehaving in one more game, in a consistent pattern of negative behavior that breaks the game's rules. Additionally, the reform card doesn't always show all the logs that led to your punishment: it randomly selects up to several logs. You might see three logs, but you also might see as few as one, even for players whose punishment stems not from a small number of egregious infractions but rather from dozens of instances of mild toxicity. The purpose of the reform card is to tell you how to reform, so it shows you an example of the behavior that prompted your punishment and explains that such behavior is inappropriate and should be avoided if you want to maintain an account in good standing.
Usually, one transgression by itself wouldn't be enough to bring such a punishment to an otherwise clean account, but the IFS works on an escalating punishment system. Breaking a minor rule, like engaging the team in useless arguments, has a minor punishment: a chat restriction. Breaking that same rule over and over again, however, doesn't prompt an endless series of chat restrictions. The severity of the punishment ramps up over time, because the goal is to eliminate the punished player's willingness to break the game's rules. If two chat restrictions don't stop the useless arguments, the system will increase the punishment to a 14-day suspension and deliver a very clear message that the continued rule-breaking is becoming a serious issue and any further instances will result in a permaban. Again, the point is to put a stop to this misbehavior. If a player is more interested in repeatedly breaking the rules than in maintaining access to their account, they'll lose access to their account.
Of course, it's possible to break major rules, like cheating, threatening people, or using chat for hate speech, and skip punishment tiers so that a clean account ends up with a 14-day suspension or even a permaban.
From Riot's support knowledgebase:
PUNISHMENTS GENERALLY FOLLOW A BASIC ESCALATION PATH:
- First Offense: 10 Game Chat Restriction
- Second Offense: 25 Game Chat Restriction
- Third Offense: Two Week Suspension
- Fourth Offense: Permanent Suspension
However, it is possible to skip to a Two Week or Permanent suspension based on the severity of the behavior in the game. Excessive negative behavior can result in a Two-Week or Permanent suspension at any time without having a chat restriction on the account.
LoL's Honor system rewards you with things like Honor, BE, and key fragments when it deems your behavior good. LoL's punishment system (the IFS, or Instant Feedback System) places a punishment on your account when it deems your behavior bad. Behaving well sometimes doesn't mean you behave well all the time. In this instance - and possibly others, since the IFS punishes based on severity and consistency, and the reform card doesn't necessarily display logs for every match where you were validly reported - you behaved poorly. Poor behavior is very impactful and thus carries a lot of weight with the IFS, so to regain Honor and avoid losing it you'll have to work on controlling your behavior. The IFS does forgive the occasional bad day, but if that kind of behavior becomes habitual, you might be greeted with a reform card telling you to rein it in, or with a permaban message if the temporary punishments haven't worked.