Simply: because the carriers want to be able to play phone manufacturers against each other like the good old days. Android alone hasn't given them as much ammo against Apple as they'd like and Android itself may be facing consolidation.

There's a real chance that consumer acceptance of the incredible range of Android phones is predicated on their belief that the Android brand makes those phones largely comparable, if not compatible.

So if manufacturers go a more Amazon-ish route with Android in retaliation for Google's buying Motorola, it's entirely possible that consumers may not be willing to follow the smaller outfits. And that would mean a further concentration of sales under the handful of manufacturers that do have some individual pull with consumers (say, HTC and Samsung). Which would only exacerbate the carriers' problem of not-enough-phone-manufacturers-at-each-others-throats.