Because sometimes they do like what this tank continued to do, stand around and insult people instead of actually tanking. And when they do tank, not taking aggro from the healer, letting the healer die, dying themselves, then spending 10 minutes on the ground telling the healer they're reporting them for not healing them. Then getting kicked and coming back and repeating.
If someone's intent is to grief by not allowing you to finish the duty, "just finishing the duty if they return" is not an option if that person is crucial to the run. In the end, we had to abandon the duty in my run. I've never seen a game that allows someone who is kicked to come back in multiple times.
I've been kicked wrongfully before after contributing, it sucks, but I still think one person having the ability to completely cripple a run by returning over and over is far worse.
If SE would enforce vote kick abuse as a punishable act instead of just claiming it, we could have this "kicked from a run, can't get back into that run" thing. But alas, they do not punish people because of "differences in playstyles" as mentioned by a GM, AKA anything really. Griefing could be someone's playstyle after all.
