That's true, but the paid staff is on the end of the content producers in exchange for having their advertisement placed next to the videos.
Also, Manwin actually charges content producers a 'content rate' in order to use their tube-sites for traffic generation. In the beginning, most tubes had unauthorized full videos which would draw in a user base, but were succeptible to DMCA take down. Now with their premier place in the marktplace, Manwin seeds their tubes with either their own content or requires content partners to produce 1 full video per X weeks in order to remain a featured partner.
As for clips... those are freebies given out to affiliates, I have a hard time believing that everyone doesn't use bots to upload that to the Tube-sites.
I'd guess that the tube site is getting some sort of service fee for allowing the sites to have a "premium" service, as well as some sort of kick back depending on the conversion rate from leads generated by the content (why the premium videos are generally listed higher).
If you think that 1) the videos are legit and that guy just happened to stumble across those two girls in the woods, then you're also going to also believe that 2) most of the content is amateur. - It's a sad truth that the industry preys on the weak minded.
I'd say that a fair amount is self-uploaded (via content bought from providers, or produced specifically for the site), but a majority is uploaded from other content houses to try to promote themselves. Think Free to air TV... average crap shows, filled with 80% advertising.
I think that they view "amateur" sites as a 'fantasy' portrayal of reality like television. Yes, TV shows and adult sites don't try to break the 4th wall but that's to be expected in a drama.