I'm curious what goes through the minds of artists when they decide to put censorship lines like this on their pictures. It's clearly not to make the picture safe for work, since too much is still visible.

RideTheWalrus said:
It's clearly not to make the picture safe for work, since too much is still visible.

That's a pretty astute observation. Article 175 of the Criminal Code of Japan forbids distributing "indecent" materials, and the general solution for most most pornography in Japan is to just disrupt the depiction of the genitalia in some way. Over the years, Japanese pornography has been pushing the envelope further and further back from a perfect block-out to something that'd barely count as a c-string, though the actual accuracy of the depiction (and whether or not it is static or animated) probably contributes to the determination. In any case, imagine if the work caught flak for some (other) reason - having no bars would be seen as a clear violation of the law. That could also blow back against any services that support the work, e.g., Pixiv, as if they were culpable for not policing it. At the very least, with the bars, the work would just get the lesser condemnation.