there are some mainstream-controversial topics that get discussed in that way here, with that sort of social norm, and I expect that in some communities the opinion of LW is tainted by those discussions in the same way you discuss

One difference is a different degree of taboo. Another one, I suspect more important, was the timing. The controversial topics didn't start by someone posting a full article out of the blue. They first appeared as comments in other articles, somewhat related to their topics. Only later someone would write an article about it. And at least I didn't have an impression that someone is on LW only to talk about the taboo topics.

In other words, if you want to talk about controversial topics, don't start by shocking everyone. (Unless it's a "door in the face" technique, when the shocking article gets heavily downvoted, but then people feel kinda guilty and become more tolerant in the discussion.)