The upload buttons of those sites seem to be there mainly to create plausible deniability. If any content creates trouble which cannot be made to easily go away

The upload buttons of those sites seem to be there mainly to create plausible deniability. If any content creates trouble which cannot be made to easily go away just by taking it down, the site owners can claim they did not upload it themselves and that they don't have any records of who did. Or they could also create fake upload records if they want to stretch things a bit further. In practice, I think it's a very safe bet that 99.999999% of the documents on those sites have been obtained through scraping other sites, and are not user-submitted content.