Mobipocket has long had a policy that dictated that theirs could be the only DRM scheme on a hardware device. Long before being purchased by Amazon. True, it hasn't changed since they were bought out, but don't lay the blame entirely on Amazon's shoulders (technically, they abide by the same legal restrictions as any other vendor licensing mobipocket's software for their device).
I would not be surprised, actually, if Adobe doesn't have the same clause in their license for hardware vendors, nor if any other vendors don't do the same - after all, you want to lock a hardware vendor into using your format, since that affects future income. Who would consider upgrading their hardware such that all current customers lose all their purchased content, after all....