The problem is not the hardware: the hardware will read Free books just fine. The problem is that Amazon wants to sell you books that will only work on their platform, since those books subsidize their hardware. (Oddly, not much stops you from buying books from other stores, except that those stores all use different DRM. It's like heard immunity or something.)

Publishers are also hesitant to offer their books in DRM-free format because book piracy would otherwise be rampant. (The fact that there's a torrent for every Kindle book ever just goes to show you how effective DRM is. Once again, it's another DRM scheme that only hurts legitimate users and publishers, but the lawyers tell everyone otherwise and so everyone is happy. It would be funny if it weren't so sad.)

There are some publishers that just offer ebooks as PDFs that you download from their website. My book is like that (though it has a PDF password that your reader can just ignore), and some other books I've bought are like that. "Programming in Scala" is just a pure PDF download after you give them your money. It's a nice system and the rest of the industry will catch on soon. For now, publishers seem to enjoy working with middlemen that take a 30% cut of the author's revenue, but people are getting tired of the middlemen and with that, DRM will slowly fade away and open ebook platforms will be the norm. Just give it ten years.