But the license explicitly states that you don't have to, so there is no legal nor ethical reason to. Claiming that there is, is nitpicking at best. I personally don't like CC0 and definitely wouldn't use but I can't stop others from using it. It's too bad, but that license does kind of blow the whole "it's unethical" claim out of the water.
In a way, the CC0 seems like it was purposely designed to end the whole ethics discussion. We just can't claim something like posting a piece of art is unethical if it was expressly permitted by its author with no unclear terms. Of course it sucks and it definitely undermines the moral rights clauses of copyright itself.
I don't like it any more than you guys do, and I'm saying that the CC0 really should be legally contested for that very reason, that it actually undermines copyright law. If it is left unchallenged, we'll very possibly end up with no legal or moral stance to stand on.
I'll add that in Finland, the CC0 isn't actually legally binding. Here by law, we can't just denounce or sell our moral rights ("isyysoikeus") to an artwork, but only the monetary and/or publishing rights.
"Nimeä ei saa poistaa teoksesta, ellei tekijä halua pysytellä tuntemattomana."
Unless the author wants to keep their name unknown (anonymous), the "fatherhood" (that is the maker's name) of the artwork must be kept alongside the work of art.
Not being legally required to do something is different from having no ethical or social obligation to do it.
My position in this ethics debate is in need of clarification. There are three parties here: the author, the publisher and the audience, out of which only two, namely, the publisher and the audience are relevant in case the work is in the public domain or published under CC0.
What the unethical thing that I'm talking about is is lying by omission for monetary gain. Because it is normally tacitly assumed that any work published without attribution is a creation of the publisher, failing to mention that the work is not one's own is unethical - in the context of blog posts on Steem. That the work was published under CC0 or in the public domain only takes the author out of the equation.
Nobody is saying it is unethical to post a piece of art if the owner of its copyrights permits it. What I think is unethical is misleading the audience into thinking that the work is one's own. In my opinion, it is perfectly fine to post images, video, texts or any works of art created by other people if a) copyrights do not forbid it and b) one does not attempt to take credit for them. Taking credit for someone else's work and benefiting from that is the issue here in my opinion. That is an issue between the audience and the publisher in this case.
In some cases there is no dilemma if the work is well-known enough for it to be unlikely to be assumed to be an original work by the publisher.
But I don't think anyone has the moral right to do a simple Google search for photographs in the public domain or published under CC0, publish them in a blog on Steem without attribution or mentioning that the photographs are not one's own work, have people praise one's photographic skills and upvote the posts with abandon.