If you've ever hosted a potluck and none of the guests were shilling junk products, congratulations! You've achieved what some of the most valuable companies in the world claim is impossible.
Nobody thinks big tech companies are OK with spammers just because their moderation of spam is imperfect. At the very least, they want people shilling junk on their platforms to pay for ads, yet none of the big platforms are spam-free. Federated systems aren't inherently immune to abuse; email spam is the original spam. Similarly, the presence of Nazis on the biggest platforms doesn't imply that the owners of those platforms are happy to have them.
Everybody with some crap to push, whether it's commercial spam or Nazi ideology has reason to look for the biggest audience with the least effort. Most of them aren't going to waste their efforts targeting Mastodon, Lemmy, Matrix or the like right now. I fear if these federated systems do grow popular enough the existing moderation tools will be woefully inadequate and most servers will switch to a whitelist model.