You can email a pdf to yourself and Amazon will convert it. You can also just add a pdf file to the kindle reader for iPad (I haven't tried on an actual kindle device).
I don't have an iPad. I did know about emailing the PDF, and now Amazon has new software that, upon installation, allows you to right click on a PDF file and choose "send to kindle."
Even so, I think it's self-defeating for an author not to make her e-book available through Amazon's kindle store. For one thing, those PDF files don't always look good on the kindle, even after the conversion process.
Secondly, the kindle store is frequented by a lot of readers. Many people would see her book there, and Amazon's royalty rate is considered generous relative to the rest of the industry. From that I understand, in the US, the majority of ebook sales happen through Amazon. I could be wrong about that though.
The one understandable concern I could see having about Amazon is fearing that someday they'll put the competition out of business and then, when they are the only game in town, their royalty rates will plummet.
Regardless, her decision risks irritating fans of her work -- and so does the excessive price IMO. The cost per recipe is really out of hand, especially when you consider that a book like Robin Robertson's Quick Fix Vegan, with many more recipes, can be had for $7.39 in right now -- and in kindle format, too.