detecting porn, presumably.
That type of check should become standard in a short amount of time for all communities that accept photos (that isn't meant to be general purpose, eg Imgur).
I checked out Facebook Marketplace a few times since the launch and and everytime I'm just overwhelmed by the sheer amount of better or worse counterfeits.
On top of that I start getting notifications more counterfeits, for a short while I reported them but after a while it felt pointless and now I just ignore the marketplace tab.