Reddit used to be like that as far as I know, and though there's plenty to criticize I don't think Reddit was intentionally set up to become a breeding ground for violent racists.
8chan's policy about spam is very different from how a policy against racism would look. What it in practice means is that if someone posts the same suspicious link on a hundred boards they may be hit by a global ban instead of leaving the mess for individual board owners to clean up. Even very principled proponents of free speech usually don't mind that.
There's no guarantee that spam will be cleaned up, it just doesn't get the protection of it being up to the board owner. For a long time 8chan didn't even have a functional anti-spam system. It's a practical matter foremost.
A rule against racism would be much more thornier. It would have more edge cases, and it would have to be enforced more consistently. It would be a lot more work and it would be more arbitrary. And it would go against the sensibilities of the original founder, who's a libertarian (I don't know if he's racist).
8chan wasn't created to be a breeding ground for violent racists. It was created to be a make-your-own-forum website with minimalist rules. For the first year, before gamergate drove more people to it, it was just that. In July 2014 an innocent roleplaying board was the most popular: http://web.archive.org/web/20140701141242/https://8chan.co/b...