One thing that amazes me from some vendors of digital products is their refusal to let customers download the file at a later date. This is a no-brainer in my mind, especially for booksellers since their products are small and the cost of this service is negligible. Amazon gets this, Kobo gets this. Heck even the purveyors of Humble Bundles get this, even though you can buy their stuff for a pittance and can involve quite large downloads (i.e. cost of service is non-negligible). Amazingly there are quite a few companies who don't. Some of those companies sell software, and others sell books.

I also had a good experience when replacing my Kindle 2i with Amazon. I didn't know whether my first defective screen was my fault or their fault. Called them up. They replaced it, no questions asked (at least not of the sort to assign blame). I'm guessing that they realize that it is better to swallow a bit of a loss and replace the device in order to keep customers coming back for ereader and ebook purchases. And they're right. The only reason why I left them for Kobo was because libraries use ePubs (well that, and Kobo's devices are more open).

The opposite case are vendors who refuse to solve even the simplest problem, or even accuse their customers of doing something underhanded. (Ahem, Steam.) Those companies will never see a penny from me again, and I'll continue to use the services I bought without a regard for the costs to them. Ditto for Apple/iTunes, though they seem to have fixed some of their policies in the meantime. Still, I haven't bought digital products from them or through them in a very long time.

So yeah, consumer trust of a vendor is a very important factor here. It may be highly individual, but considered collectively a lot more people seem to trust Amazon than other vendors. So it makes sense for authors to link to Amazon. They'd be foolish not to.