Some people are probably doing it as a political statement about how it shouldn't be a huge issue, probably? (And if you're in a place where you can afford not to care, then go for it.) Some people may be actually camgirling and posting free samples - I don't know for a fact that it happens on DW, but afaik if it's legal it's not against the TOS, and a really good reason to mark an entire blog NSFW.

And some people may be posting nude photomanips of fictional characters or other nudes that are not them - that's probably most likely. (I think at some point I posted some vintage nude pinup postcards from the 30s we found in my grandfather's house, for example. I have no idea if I'd've locked those or not. And maybe scans of my 70s Chippendales playing cards?)