With Readlists you are essentially publicly sharing an eBook that contains a writer’s content — content that was never granted permission to be redistributed. In Instapaper (as far as I know) you can’t grab an article list from another user and package an eBook that you can then download.
Readlists is a service to allow people to completely avoid reading a content producers site, allowing them to repackage and redistribute their content all without permission.
While lots of services exist to re-package content to make it easier to share or read, re-publishing it is stepping over the line.