My understanding is that it's Amazon wanting to set the price at which it buys ebooks from McMillan (where by "buys ebooks", I mean "agrees to pay for each digital copy sold"; honestly in my view the distinction is obvious and irrelevant to the discussion). It's not that McMillan wants to dictate selling prices to Amazon; that would be stupid. It's that McMillan wants to start charging Amazon more for new books and less for old, and Amazon wants to use its market dominance to say, "Nuh-uh. We pay this much or not at all."
I don't have insider knowledge on this; nothing I've read has bothered to hash it out. My guess is that just doesn't occur to people to bring it up, because it's obvious to them. I mean, nothing else makes any sense to me.
Personally I don't see the big deal with charging more for new books and less for old books (I would much rather pay $14.99 for a new eBook than $29.99 for a new hardback). I believe that Amazon might have some valid points (I've not heard of any thus far but they might actually have a method to their madness) though right now I just think they're abusing their power on the eBook market. I mean everyone wants to make money so if McMillian wants to charge more for the digital copy of new books and less for the old books it would still even out to the same amount as if they sold all their books for the same price.