From my mostly uninformed POV, a company can do whatever they want with respect to this and be on good ground, making it an arbitrary decision with little basis in actual objectivity.
Each case is different and the details drive the decision.
Longer version: Not sure how Amazon deliveries by UPS are in the US but in the U.K. there are carriers which offered far better delivery service than what is known as Amazon Logistics here in the U.K. Once Amazon started using Amazon Logistics, the level of service dropped significantly. False deivery attempts were common and Prime Next Day were not happening next day 3 out of 5 times. I cancelled my Prime membership.
After about a couple of years I have resubscribed to Prime since it is now much better value AND Amazon Logistics have noticably improved. Next day and even Same day deliveries are indeed happening.
They didn't need to setup Amazon Logistics and suffer poor service quality. There were near perfect couriers (for e.g. DPD they even offer tracking your courier driver on a map in near real time).
This is vague.
Running on the cloud is always more expensive than running your own infrastructure past a certain size and always provides less predictable performance.
It's actually about whether or not it would cost more, over the long term, for a company to develop such capabilities themselves, or to outsource it.
Generally, due to economies of scale, a big, established provider will be able to provide such a service much cheaper than it would cost you to run an equivalent service.
However, it depends on how much that provider is charging you, on top of their cost of sales. It's pretty basic in that if it costs more to do it yourself, it's easier to let someone else do it. Especially when you take into account the considerable R&D costs that it would take to provide an equivalent service. Google's services would, I'm sure, be highly developed. This statement says as much, in that it claims Google has services offered by no other company.
So no, it's not arbitrary.