Indeed, a warning that it may contain illegal content would be about as sensible as the standard "Warning: may contain nuts" label on a tin of nuts.

What is illegal in a download of an online drug marketplace? Are pictures of drugs banned where you live?

It's a general black market, not just drugs. For example, one of the sites described on that page is PEDOFUNDING, "A crowdfunding site for child pornography." Now the dump isn't supposed to contain any images, but it's hard to be 100% sure. In any case, whatever risk there might be seems to be clearly implied in the name and description there.

FreeeOW, that's what I get for skimming the list I guess.