Great question, but I don't have the answer, unfortunately. Maybe because they consider AMD to be worthy of low-end devices, I guess... and probably because it may be easier to integrate their platforms this way.
problem with amd is despite so many claims on how good the built-in graphics, they, or notebook manufacture never have any confidence in the performance, and pair it up with a dedicated radeon. if they can release i3 notebook with lousy built-in graphics.. why cant they release an a10 notebook without dedicated graphic, save cost, power consumption, and plug in a good speaker and ips lcd?