Opera Mini is likely not causing any uproar because they're a 2nd or 3rd tier browser. They're not a 1st tier browser, and they're certainly not the default browser on a 1st tier platform.
Amazon, however, is making a play to be a 1st tier Apple-style comprehensive platform for mobile devices, media, and applications, and they're shipping "Silk" as the default browser.
Opera Mini is indeed the default browser on several phones. It's one of the few discernable ways that Opera makes any money. Granted, nothing as popular as the Fire is likely to be.
Amazon, however, is making a play to be a 1st tier Apple-style comprehensive platform for mobile devices, media, and applications, and they're shipping "Silk" as the default browser.