Give me one good reason not to leave games?

Just left a game. The reason is simple, dude was afk, 3 premades (Probably) refused to remake. Game turned into a stomp, and 17 minutes in I decided to leave the game. The dude that was afk comes back after 10 minutes and proceeds to flame me for not helping him. What kind of cancerous community is this, and why is leaving games punishable?
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