In sports at all levels of play, it is a valid strategy to get in your opponent's head to demoralize them.
For example: when I played soccer, I was a goalkeeper. On several occasions, I had opponents realize I was very good at diving all over the place and making unreal saves, but I wasn't very big. So they would put the biggest kid on their team on offense, and any time there was a contestable ball I was going for, they would have him run straight into me. I got fucking wrecked sometimes. But even when I ended up sprawled on my back, winded, concerned about a concussion, I never stayed down for more than a minute. I wasn't going to give them the satisfaction of beating me through cheap tricks, even if it hurt. And you know? My team usually won those games, because the other team focused so hard on beating me down that they forgot about playing the game.
Is league competitive? Sure. Does everyone treat it that way? No. Either way you choose to treat League, though, there is a logical fallacy in leaving:
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If you don't treat league competitively, it's about more than winning or even doing well to you. Therefore, being camped or focused shouldn't be treated as something worth leaving over, because it's just a variation on gameplay. You can't play to win or do well every time, and you should not base your enjoyment of the game on winning or doing well every time, because you statistically cannot win or do well every time.
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If you do treat league competitively, you should want to win. You can't win if you AFK.
I don't condone AFKers. Neither should you. I understand emergencies, connection issues, etc., but willingly leaving something you've committed your time to is a weak reflection of character under any circumstance, especially if the going gets tough.