I do believe that no DRM would indeed benefit Amazon, just like it did Apple. To carry on the iTunes analogy, the sales in the iTunes Music Store really exploded into its current success after the DRM had been removed, not before.
Indeed, what makes Amazon/Kindle Store so successful and nice to use is the customer/shopping experience and the amount of content available. I've already got a Sony Reader for a good while, and it bugs me a bit that I cannot read any content bought from the Kindle store in it, even though I can read it in my iPhone, computer and so on.
I recently bought a Kindle reader, too, just to be able to (comfortably) read the content from the Kindle store, content that was not available elsewhere. I would have bought the content, anyway, had I been able to buy it directly into my Sony Reader.
In fact, I probably would have preferred the Kindle store, anyway, had it been available in, say, the Reader Store, too, because of the better and easier shopping experience at Amazon.
Surely Amazon are making their actual profit from the sales of the Kindle content, rather than the sales of the Kindle devices. I don't think that the profit margin in the devices is that hight to begin with. Apparently the Kindle devices sold at the lowest entry level prices, as well as the Fire (inside the US) are heavily subsidised by the advertisers, so the profit coming from the sales of those depends heavily on the content the customers will buy.
What makes the Kindle store so appealing is not only the size of their title catalog, but also the ease and simplicity of buying content into your reading device, all over the world. Most of the other online stores are much more restricted. Reasonable pricing is another factor. No other supplier manages to deliver the same experience quite as well. Therefore DRM is irrelevant and more of a nuisance than a necessity. Without it they would sell even more than they're selling now.