People are asking about Japanese law on this, so I'll write what I know. I am not a lawyer, just a person who cares about copyright. Japan does not have fair use, instead it has a number of specific exceptions to copyright. For example, you can take photos of things semi-permanently installed in an outdoor area and use those photos for non-commercial purposes, even if you photographed a 2D artwork that might have cop…
What's the legal status of retweets? From a technical point of view, the user is not making any copies but only referring to another post, and it's Twitter who creates the preview. Furthermore, how has Twitter ever been able to operate in Japan in the first place, if they have not made any concessions by disabling features that are considered to be in violation of local legislation?
If you are unfamiliar with Twitter, you could consider a user's timeline like their "home page", since they control it, and an RT could be considered a form of "reposting", because you put it in places it had not been before.
I don't think that's a reasonable line of thought, but it is consistent.