Every other ePub3 distribution system in the world has some or all of these limitations. . .

Not all distribution channels that accept EPUB3 deliver actual EPUB3 files to consumers, and not all EPUB3 files can be read on different devices and software programs. Some content providers store EPUB3 files on their servers to enable online “cloud based” reading in browsers (Safari Books Online for example). Other (Amazon Kindle for example) convert EPUB3 files into their own proprietary formats and don’t allow consumers to directly “sideload” EPUB3 files.

Still others (such as Apple iBooks) apply a proprietary DRM – “Digital Rights Management” technology that encrypts the content of an EPUB3 file such that it can only be read on the vendor’s own software and can’t be “sideloaded” into other EPUB3 compatible reading systems – note that while Apple doesn’t make their FairPlay DRM available to 3rd-party reading systems, even on Apple devices and computers, Apple iBooks does allow users to “sideload” non-DRM protected EPUB3 files that come from arbitrary sources.
Only the Embellisher (TM) ePub3 eReader has no such reader limitations.