Ultimately that will always be the case. This is just one example, but there's a million ways things can end up mirrored from various services. There will be torrent instances and porn instances and whatnot. With the way the fediverse works, you have to protect yourself from spam and bad content. The problem is the inability to filter it out, which thankfully Lemmy 0.19 now has: you can now block whole instances as a user.

But also generally, don't use All. It will always have random crap you don't want, especially as some instances use a bot to subscribe to everything. My test community has 74 subscribers, 72 of which are those bots. This means my random test crap ends up on All of lemmy.world and there ain't much I can do about that other than marking it NSFW so it doesn't show up to guests. The All listing sounds appealing at first and maybe it made some sense on Reddit, but on Lemmy it kinda doesn't work. Especially when non-english communities will take off, a good chunk of All may end up being in a language you can't even read.

Honestly a better fix for this would be custom feeds, or a way for admins to curate the contents of All without having to pull out the nuclear weapons and defederate.

I can see why some people would actually want a readonly Reddit mirror. Like, maybe there's a community you used to follow but never write to but don't want to have to use the Reddit app for. I understand why that'd be a minority of users, but clearly there's enough demand for it that it's a thing and there's even plans for implementing two-way bridging. Or if anything, tinkering with such a thing is a very fediverse thing to do. We shouldn't blame the instance for existing but the lack of tools to manage their existence.