Re: your comment above about continuing the color to the front bezel--I think there are two reasons. If you think about your phone being a content display device, this makes sense. If you're watching a movie or video clip, you want the screen to "fade to black" at the end of a scene, and in-between the transition of one scene to the next. This creates an immersive experience as you can get lost in the content before your eyes.
Black bezels also make your screen look bigger--think about older iPhones with those huge white bezels and you'll see what I'm talking about.
I actually like that both Google and Samsung use black bezels for these reasons, and I really hate white bezels.