Because in many cases no "troll" has been found at all; and discouraging people from witch-hunting leads to a more positive environment that's conducive to interesting conversations. Often the witch hunter is just salty about something someone said. For instance, thomasanders0n is evidently a big fan of Paul Graham's writings. I left my impression of their article, which was short and to the point; I personally think the article is so bad that doesn't merit a lengthy teardown. But because I feel differently, my response has been interpreted as bitter and jaded. Shrug.