This is very interesting and should make all POD (print on demand) sites sit up and take notice. For years, real artists, (as opposed to thieves and copycats), have been saying the sites need to do better 'due diligence' in preventing and removing infringements and terminating repeat infringers. All this time many PODs claimed that by responding to takedowns they have safe harbor from liability. Maybe not. Even before Gardner v Cafepress there have been holes in that theory.
With reverse image search technology, plagiarism checkers, etc, it is not impossible to do, it's just that none (that I know of) have done it yet. And it does not take special software to recognize when a member's entire portfolio is full of Disney characters. Why are such members there for years on end?
Sadly, it seems infringement is profitable, as apparently is turning a blind eye to it.
Unless you get sued, like this site has been, (and this is not the only case). Many sites Terms say they can hold members accountable for legal costs. That is a message that infringing members need to receive, too.