"Illegal" is a harsh term, I'd rather say "legally naive". There's no TOS anywhere saying things like "you give us the right to publish the comments you enter" which would clarify things but if you were to take an ordinary instance to court, you'd probably be thrown out with reference to you implicitly agreeing to have your comments published by, well, writing and submitting them. Licenses are ruled by contracts and contracts don't necessarily need written form.
Meta is a whole another thing, though, because now we're not only talking publishing, but straight commercial exploitation of your content. There's no equability to be seen anywhere, meta doesn't contribute to the maintenance of your home instance, it straight-up leeches your content to put it next to ads. An implicit license doesn't suffice for that, a written one might not even (because no equability), that's why all the corps have TOS.