Why is it always wrong to AFK

Afking is wrong, period, full stop, right? Usually obviously yes, but - real life emergencies aside - there seems to be one situation when it doesn't seem so... I had a game a few times ago - when I happened to be feeding - and I was asked by my team to go AFK. This time they were NOT flaming - rather - they were sincerily asking, since they believed that the cost of having their enemies more and more fed was outweighting the cost of being 4v5. And they may have even been right. To put it frankly, my incompetence was causing my presence to be more harmful than helpful to my own team. In this case I would say it should be justifiable for me (and other players in similar circumstances) to go AFK. Am I right?
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