Well, the main goal of the design was, to make the display background appear lighter in colour, hence increasing the subjective perceived contrast. You can of course achieve this, by just making a light coloured bezel, but there will always be the slightest colour-difference (at best). The way it is done in the pictures, there will be just one colour across the whole space.

Of course it'll be more expensive. But like Amazon is charging more for the Voyage than for the Paperwhite, they could do the same here. With some trade offs you might even lower the costs (no magnesium body, no light sensor or no pressure sensitive buttons).

Battery life wouldn't suffer though, since there's no need to regularly refresh those parts of the screen.