To rephrase that, they had no control over whether the result of their action is legal or illegal.
They did it anyway.
In this scenario, one should verify the result of one's action.
In any case, this is bonkers! But it probably feels that way because social media has conditioned us to take other people's work and do whatever we want to it and with it.
If the post office decides to use illegal means to deliver your Christmas postcard, or steals a copyrighted work for the stamp it's not you that should be punished.
> social media has conditioned us to take other people's work and do whatever we want to it and with it.
Perhaps but this is not the example with which to make this case. The material was already on the platform and they did nothing to appropriate it or profit from it.
I am not familiar with the Japanese justice system although it's known for some very idiosyncratic situations. But using common sense one should not be punished for an action taken with no bad faith, no reasonable expectation of breaking the law based on how another party decided to implement that action.
No matter what the solution is, the people who retweeted are definitely not the ones to hold responsible in any sane justice system. They retweeted a picture but had no input on how it is displayed, and have no control over whether this changes in time.