Quoted from PinballTilt:With the amount of movies they buy they could get the studio to do anything if they were willing to pay for it. Most likely they negotiated a lower price per disk if they don't come with bonus features.
What’s a rental, a dollar twenty nine? At the price level they charge, they couldn’t afford to buy the discs. For a while when the studios cut them off, they were buying the retail package to say F you, we can buy the content. But that model couldn’t be sustained long term due to their low cost to the consumer.
In other words, you get what you pay for, and that is all the studio will give you at that price.