I have a feeling that Amazon may actually dislike people selling crowdsourced material as assembled books, not necessarily copyleft books (such as Dive into Python).

I can think of no reason Amazon would be against selling crowdsourced books (besides quality control). If you have ideas, please enlighten us.

You can think of no reason besides that one reason you gave? ;)

According to the email he received, Amazon doesn't allow you you publish material you do not own the copyright to. So this effectively kills crowdsourced material unless you have a copyright assignment clause.

By your reasoning Coca Cola are a customer of Walmart because they use the shelving facility provided by Walmart.

I'm not saying suppliers aren't important, they are, but they aren't however customers.

This translates to Amazon. They are selling the rights to distribute one's books on their platform, and in return take 30%. Amazon provides a service, the bookseller pays them money. (In this case, that money is zero, but that's beside the point. In general, the seller is Amazon's customer.)