First, it should be know, the USPS has become self-sufficient and has not directly received taxpayer-dollars since the early 1980s. However it is currently borrowing money from the U.S. Treasury to pay its deficits. Its building up debt, thus the drive for the change.
Amazon offers flat rate shipping subscriptions (their Prime service), anything that costs more money to them hurts their bottom line initially until they can figure out how to adjust it later(It's an annual subscription). Just like how they are fighting this "brown bailout" which is UPS's attempt to lobby the law that prohibits fedex ground from unionizing. FedEx unionizes their workforce, and their costs go up. Then UPS can put some slack back in their profit margins. Amazon doesn't want that to happen either. Also if the cost for Saturday delivery goes up, people will be less inclined to opt in for the upgrade in shipping, and instead turn to a local retailer to get their goods in time for their weekend needs.
Netflix is only supporting the USPS change because it'll eliminate 1/6th of their delivery/processing days, and saves them a ton of money. They haven't stated yet if they plan to lower the costs of their service to reflect the loss to the customers. Of course they try to spin it as supporting the well being of their delivery medium, but in reality it again all comes down to the bottom line.
I personally like the option of a cheap Saturday delivery, and hope that the USPS can figure out a way to generate more revenue and keep their 6 day schedule.