I didn't realize losing the game in a secondhand manner wasn't a bannable offense

I just had a game where I got banned for it. Completely reasonable. The other person who I chatted to did as well I'm pretty sure. My problem is with his duo. He didn't type often in chat but sat in our jungle farming while his friend was inting and trying to lose the game on purpose. Now I didn't know not typing while passively losing the game was allowed. I want him to get punishment as well otherwise what stops others from not typing but just being disengaged from the game completely to lose on purpose. This isn't an instance where a person got caught and he had to wait to regroup or maybe he made a bad judgement call or any other league instance you can think of. He is queued up with someone who openly said in the chat log "ima feed now" and changes his entire behavior towards the game to be completely disengaged from the game. Not to mention he says something in the chat logs referring to his friend feeding along the lines of "this is his third account you really think he cares" acknowledging his duo's behavior and going about losing the game in a different more subtle way.