ELI5: Why are we obligated to censor people's identity, when we share a post/picture that they have made on an open and public forum?

I understand what doxxing is, and I agree that in a vast majority of cases it may be unwarranted and terribly damaging. But for those special cases where someone has made a racist/bigoted/violent/purposefully offensive comment or post, why are we obligated to censor their name/username?
When posting something on/in a public forum aren't we all assuming the reward/risk of having that comment/post shared?


Source.