There's a joke (can't find the comic or article, darn you Google search) that says when you make a forum with "free speech" you get one civil libertarian and 99 trolls, neo nazis, child pornographers, etc.

Moderation is essential for usable communities. These block lists are a form of moderation to make Mastodon useful. Otherwise it's pushed on to the user, many of whom will just nope out back to Reddit or Facebook, or to the Mastodon server administrators, who are largely unpaid volunteer labor.