As my mind twisted around these dynamics, it became clear that there really is no distinction anymore. All of these (self proclaimed) groups have really the same objective to find content that the market will pay for and make as much money from it as they can. That's it. That's what they all want. The only difference is how much risk they are willing to take, how they go about it and what work they are willing to do.
There is no way to generalize what one group or another is willing to do. Some authors are willing to do everything, others don't want to do more than submit their manuscript and be left alone. Some agents are willing to cull slush piles looking for the next great novel, and hire editors to help polish up a manuscript, others only want to deal with rights deals from established brand name authors. Some publishers are willing to do the hard work of creating markets, while others are only willing to push a work through their established processes. Some retailers are willing to leverage their community and the author and the work in every way possible, and others only want to hang out a shingle and wait for customers.
This really wasn't different in the print-only world - especially before digital production - except that publishers were the only ones who invested in all the processes that brought a work from concept to product on the shelves. Publishers were clearly the group with the largest risk, and hence fairly expected the lion's share of the financial return.
In the electronic world, there is the belief that this is no longer true. The risk is not as great, the need to invest in scalable processes not as necessary. This may be true for the briefest of periods, but as virtual retailers pop up everywhere in all kinds of forms, and works of all quality levels are brought to the virtual shelves, my belief is that this pendulum will once again swing in the other direction.
But, until then, we have a very disrupted ecosystem. It's interesting and exciting times in the publishing business! The companies to watch are those that are investing in scalable operations. From my perspective those companies don't fall under any of the aforementioned groups.