 How could selling their work actually harm some authors' interests?
Authors who share their works in this way aren’t selling them, and even authors with purposes higher than money depend on sales to make a living. Suppose that selling their work would actually harm their interests by shrinking their audience, reducing their impact, and distorting their professional goals by steering them toward popular topics and away from the specialized questions on which they are experts. Copyright can also be a significant access barrier. Even these authors, however, tend to transfer their copyrights to intermediaries—publishers—who want to sell their work. Second, scholarly journals generally don’t pay authors for their research articles, which frees this special tribe of authors to consent to OA without losing revenue.
 The cost to read the work could result in a smaller audience.
 It could steer them toward writing about popular topics rather than writing about their expertise.  Libraries may decide not to purchase the work because of the cost involved for multiple copies.  They may never reach a global audience.  The cost to read the work could result in a smaller audience.