One issue is that, especially with regards to “conspiracy theory” type content, minor exposure increases the risk of being sucked in. Even joking about COVID hoaxes or talking about how ridiculous they are can increase the number of people believing there is something going on, leading to real life harm.

Another argument in favour of preventive blocking is that it's hard to understand the effect some of that hate speech has on someone if you're not from a marginalised background yourself. If we want this server to be inclusive, then we don't want (e.g.) people of colour creating an account, logging in, and seeing a whole lot of racist stuff.

All the big corporate social media sites have massive teams of people deleting that stuff before it ever surfaces (although some of them don't do a great job). The fediverse doesn't have that, and it has created problems for people who have been targets of racist etc. abuse, and has ended up with them leaving (especially on Mastodon). Defederating from some of these instances doesn't solve the problem, but it's part of the solution.