The "tube" sites are more exploitative than the rest of the industry in that they don't actually pay the performers, they're simply piracy, and because small independent producers don't have the political leverage to impose an equivalent of content-ID on them they stay that way.
Your comment doesn't just apply to porn tubes but all tubes. Youtube has tons of pirated content, they take it down when reported, just like us.
You build a gigantic audience on pirated content, then once you are many people's go-to destination for porn, you can allow the big companies to have official partnerships.
They are kind of stuck with that partnership because of the size of the Tube sites audience, which they built illegally. They also CONTINUE to post pirated content everyday, which sustains that audience and prevents those companies from just making their own TUBE site. But because the pirated content posted today will be from users, and the TUBE site will eventually take it down, the TUBE company is not a bad actor in this situation.
At the end of the day, I can still watch the newest Netflix Standup special I've been waiting for, for free on Youtube. And I'll still catch the new Adult Avenger's Parody on another TUBE site for free, without the consent of the companies that paid to create those films.