Many of them already do, e.g. local booksellers. They might not be price-competitive, but if Amazon provides the linker with no incentive to link Amazon versus linking some other bookstore (say, their favorite local bookstore), some percentage of linkers will (I would guess) defect from choosing to link Amazon.
For example, I have a site with some book reviews which currently links the Amazon page for all the books, because they pay me. If Amazon terminates its affiliate program in my state, why would I continue to link Amazon, rather than someone else? I would first look into alternate affiliate programs, like AbeBooks. If I couldn't find any worth using, I'd probably change it to just link the official publisher page for each book. Or maybe to a local-bookstore aggregator like indiebound.org. Or the Wikipedia article, when one exists.
Even if I were making money on Amazon's affiliate program and it got cut I would continue linking to Amazon. For the reader using my site to find information going to an online location where they can purchase the book easily will help them come back to my site.
Especially if I have diverse traffic from around the United States or bigger linking to a retailer that is more likely to ship to my readers will let me keep their eyeballs.
Like many here, I have many book reviews myself that link to amazon. I link to amazon because that's what's most convenient for most people. I do not use an affiliate link because it's a conflict of interest. In the past magazines would have honest product reviews that sometimes were negative. Nowadays nearly every magazine only has glowing reviews and no negative ones. Editors tell writers that bad reviews will result in lost advertising revenue since publishers/manufacturers threaten to stop advertising in magazines with bad reviews of their products. Contrary to this I have many reviews that rip apart products and point out their numerous defects. I also have reviews that praise good products. Perhaps some people who read my reviews notice this and find the integrity refreshing or bizarrely archaic. In any case I certainly don't need the pitiful income from affiliate links, it makes more sense for me to just use a plain link.
