(ETA: This paragraph below is diametrically wrong. Sorry.)
AFAIK in the USA, registered copyright is necessary if you want to bring a lawsuit and get more than statutory damages, which are capped low enough that corporations do pre-register work.
Not the case in all Berne countries; you don't need this in the UK for example, but then the payouts are typically a lot lower in the UK. Statutory copyright payouts in the USA can be enough to make a difference to an individual author/artist.
As I understand it, OpenAI could still be on the hook for up to $150K per article if it can be demonstrated it is wilful copyright violation. It's hard to see how they can argue with a straight face that it is accidental. But then OpenAI is, like several other tech unicorns, a bad faith manufacturing device.