First of all, it's the appropriate price for the service provided. Full stop. Publishers don't create anything; they facilitate. Facilitation is not WORTH the lion's share, even if publishing isn't possible without the lion's share. But that takes us to the second point, which is that the research I've done shows working models of the hosting service operating with as little as 2%, given appropriate volume, technology (bleeding edge video codecs and image compression; that's your expense so it gets the research), and scope (youtube serves the globe; targeting is the startup's best friend). But there is also another option of content creators hosting their own content and providing it to the site via a stream api that can handle content dynamically (for working with CDNs, and whatnot). This would, incidentally, prevent content creators from tying up all of their online value in a single platform that can unilaterally shut them down (even if it is a burden; at least it's an option).
But I think the answer you're looking for is that the publisher gets to make money as a facilitator, rather than a program scheduler. The model doesn't make users the customers, it makes content creators AND advertisers (together; doesn't work without both) the customers. If advertisers want to work with a content creator on your network, and they want that content viewed with your network, then they will have to make a deal to join the network. That's passive revenue for the site, on top of the percentage of the ad buys. Essentially, you put the "no free rides" subscription model on the advertisers, and then let content creators decide who gets free rides for THEIR subscription models.
On top of that, there's a whole other revenue stream in developing "networks" and selling "social" functionality to advertisers and content creators that get value out of interactivity (ex: monetizing the inclusion of advertisers in "best value" ad-buy deals that target a "content creator type" like "these ads go well if you provide cooking content") . And we haven't even talked about "whales" and monetizing the customer base with twitch-like engagement mechanisms. I cringe at the thought of some of it, but if the creators can make money off of it, more power to them! And a little percentage of their windfall to the site! ;)
Of course, there's always a new way to make a buck, and there's some very deep pockets in advertising, so I don't fear for the innovation that could go on, in a site run as described.