This appears to be a new requirement by Google as part of its Google Mobile Services agreement for Android phones. According to Geek.com, the mandate must be followed in order to have access to Google Play. Other sources on the web say it's also required for access to first-party Google services like the native Gmail app. Not only must the "Powered by Android" tagline appear on new handsets, Google even has a set of guidelines pertaining to where it's supposed to appear.
It's not clear why the policy has suddenly been put in place, though we can surmise it's an attempt by Google to get the Android name out there more than it already is. Google might also be looking to remind users that their device is running Android even if it's so heavily skinned and customized that you can hardly recognize the OS..
Google isn't asking a lot here, just a bit of specific branding during the boot up process. It will be interesting to see if the same policy extends over to tablets, especially ones that dual-boot with Windows.