Ad-hoc appeals, not principled application of how things actually work. Visiting a video hosting site anonymously, and being sent a video, is not "piracy." Even training on Disney DVDs is transformative and so falls under fair use. No significant portion of a vast original corpus is recreated verbatim - in this case, ideally none of the corpus appears. The goal is to produce nothing like these videos.
Or if this is for classifiers instead of generators, nothing appears, because Meta's not publishing anything. They're looking at porn to make a program that goes 'yep, that's porn,' to remove any hosted porn.
So it's not a competing work, it doesn't substantially reproduce the original work, it's not even the same medium, and if anything it's protecting the commercial value of the original work.