In general, Company A wants to control the entire experience of using service A, promote the features of service A in exactly the manner they wish to promote them. With a unified UI there is always a compromise: they cannot have all the buttons in exactly the places where they want them to be, there needs to be some kind of balance between all the services. So there is a rather strong logic. Considering the overall user experience of the device it's perhaps an unfortunate logic, if you choose to see it that way.