Countries have laws both protecting people who host content provided by third parties and imposing certain responsibilities on them when they become aware of illegal content hosted on their servers. Some of them, like Germany's NetzDG impose specific procedures for reporting (though no Lemmy server is large enough for NetzDG to apply). US laws about child pornography, for example are very specific about removal and reporting requirements, come with a risk of prison, and can include things that are legal other places such as cartoon drawings.

Laws don't need to specifically address whether the content arrived via a federation mechanism or a user uploading it directly, only what a server owner must do once they're aware of illegal content on their server.