Some dealerships might feel this way but are not looking at the bigger picture.

GM is showing the dealerships that if they aren't going to buy the cars then Carmax will. Either the dealerships are getting the business or the customer that is looking for a like-new, low-mileage vehicle will go buy it somewhere else. The vehicles are not owned by dealerships anyway. The dealerships may have sold them new but now they are owned by GM Financial, who can do whatever they want with them.

They have used the traditional auctions as an outlet in the past and are now funneling the business back to the dealerships. The dealerships are not required to sign up but it doesn't cost the dealerships to participate so I imagine with a few more months and all the dealerships will be on board.