Isn't the point that some bidders will not know which it is when they make the bid?
RTB bid requests have support for the normal auction based bids but also indicating possible private marketplaces which may come into play depending on the exchange and seller, meaning that the highest bidder will have indicated to the world their price, but not won the auction, so they may (erroneously) conclude next time to try bidding higher, leading to price distortions.
I suspect a large proportion of advertisers still believe the whole thing is a nice transparent fair auction process, and have no idea of how convoluted it has become.