(b) The publisher's other retail markets (Barnes & Noble, Wal-Mart, other distributors and bookstores) would crap out a literal brick, which they would then apply to the publisher, if the publisher started competing with them.
You can also put non-amazon books -- in formats such as pdf -- on kindle. It's not as simple as buying through amazon, but it's very doable. Further, everyone who owns a smartphone or a tablet can just download a reader app.
One publisher who I find particularly reader-friendly is Baen. If I buy a book from them, it goes into my account there so I can download it at any time (and any number of times) in the future, in a wide variety of formats. Since I do my reading on a kindle, I click the send to kindle link and it pops up next time I turn on my kindle's wireless.