> I can't imagine a situation where someone has beliefs that do not affect his actions in any way.
That was an extreme example used to illustrate the point I was making. Racism is also only an example; there are plenty of other beliefs that are bad for society. Personally, I suspect that sexism is probably far more prevalent on Reddit than racism, for example.
Social stigma can prevent one's beliefs from affecting one's actions, or at least reduce the effect of those beliefs on one's actions. If it is shameful to be perceived as a racist, racists will attempt to hide their racism by not acting in racist ways toward others, at least when other people are looking. This does not obviate the effects of unconscious biases or eliminate racism, but the net result for society is fewer overtly racist actions, and that's a good thing.
If racism becomes acceptable again, racists will feel freer to act on their beliefs, which will make life worse for the targets of their racism. So, there is a case to be made that if people repress their beliefs only partially, society is better off -- not as much better off as if certain people repressed their beliefs fully, but still better off.
The danger of allowing racism to flourish in online communities, especially ones as large as Reddit and Twitter, is that it re-normalizes racism and makes it seem acceptable to be a racist. It can be more than merely acceptable -- it can even be a contrarian badge of pride, justified as a rebellion against political correctness. (That kind of justification for it is reinforced when anti-harassment policies are not uniformly and consistently enforced, because it can make moderation appear to be arbitrary or politically motivated.) When that kind of thing happens, it makes society worse.