From a purely grammatical/logical point of view, the statement: "when all existing ebook editions [...] contain access controls that prevent the enabling [...]" would mean when they prevent the features in some software, regardless of whether or not it works in some other software. In particular, if I have a reader that has text-to-speech for DRM-free books, and some book is only available with DRM, this case would be included in the above statement.
Of course, in legalese grammar and logic rules do not apply.
