This move is uncomfortable but necessary. The two businesses are different at every level (supply chain, cost structure, technology, user experience, etc.), and the DVD business is on its way out over the nest 3-5 years.
Better to disrupt yourself now than wait for some upstart to do it and react.
But if Amazon's DVD business, or selling organic tea, doesn't perform well in the long run, they'll still be in business. If renting DVDs or streaming movies doesn't perform for Netflix, they're out of the game.
Amazon built the company around having a million services and products. Netflix built the company around having one service, and then around having two services.
They're a generic store. Netflix doesn't want to be the equipment of an online department store. They're a boutique shop that has gradually realized they have two competing interests under the same house, and would rather have two shops side by side than one.
Better to disrupt yourself now than wait for some upstart to do it and react.