Even if it's the paid version, the prompter had it make the infringing content for their own use. And it's only going to be anything more than equivalent to someone making their own sketch of Batman if the prompter reproduces and distributes. It's not like it's illegal for you to tell your artist friend to sketch a Batman either - Comic Cons and such are full of that.

Midjourney is returning copies of copyrighted works while its Terms of Service state that “You own all Assets You create with the Services”.