Is it okay to disregard teamfights and go for easy kills in order to get Rank S?

We had a Master Yi jungle that only ganked when the enemy was either easy to kill or when they were low health (from killing us that is). He never missed a dragon and stayed in the jungle plenty of time. Late game he would wait off for teamfights to end to start picking easy kills if he could, always playing very safely. However he got to the point where he just straight up sat on the fountain, saying to /all "Mid is open, finish" so he would keep his score. Is this punishable? Aside from the afk bit (and the flaming he was doing in-between but that isn't important to the question) Is it fine to play safe to the point you lose the game? What do you think?
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