How we can differentiate between the Kindle as a dedicated book device and other electronic devices such as PCs that have text to speech as a feature, or the future smartphone, is highly questionable.
When you change the typeset font, size, pagination, are you breaking rights?
Does every file format need to be specified and rights against it agreed in advance?
When I read a book on a PC and use text to speech is that ok? If I read a book out loud but through a microphone and amp is an infringement?
Remember all the rights that the physical copy gives you today that the digital copy doesn’t.
What is fair use and is this about rights or money? If the later how will you determine the usage on a downloaded file?
If you merely slap on a supplemental charge for a feature you may never use, can’t monitor and is accepted by all all poor file quality, is that what we call fair?