That's the difference between abusing a feature with a too extreme form of the intended use, and abusing a feature with a too extreme form of an unintended use.
Forums are about discussion, exchanging opinions and arguments. Drawing the line here about what is too agressive and what is not is fairly difficult. Ultimately, if you don't want to expose yourself to potentially agressive discussions, don't come to the place where people are meant to discuss things.
The game, on the other hand, is meant for playing the game (duh). Even the chat is essentially meant for that. You can strategize and communicate otherwise difficult or ambiguous things to communicate. Discussions are possible, of course, as long as they're polite. But the chat isn't meant for that. And it's definitely not meant for judging other peoples' playstyle or skill. That means Riot doesn't reallyhave to care about your intention when punishing you. You're already using it for an unintended purpose. If you can't do that without offending other players, you should either not use it for that unintended purpose, or you will get punished.
Ultimately, you can somewhat (though not entirely) draw parallels to the difficult of punishing intentional feeding. That, too, is just an extreme form of something that already exists and is intended to happen: People dying to the other team. As a result drawing the line is more difficult. Who's intentionally feeding, who is just bad at the game? Just like on the forums the question is who's intentionally being insulting, and who just has this (bad) style of agressive discussion.