I believe companies are paying them for access to the best students of specific courses (for recruiting).
For example, the Princeton Algorithms course says:
.. Coursera will maintain limited data regarding student progress and performance in this course and, with your permission, provide authorized third parties with access to such data.
Given what recruiting fees are, I'd imagine this could work quite well. It is unclear how well it will scale, but if they can keep their costs low enough it may not need to.
For example, the Princeton Algorithms course says:
.. Coursera will maintain limited data regarding student progress and performance in this course and, with your permission, provide authorized third parties with access to such data.
https://www.coursera.org/course/algs4partI
Given what recruiting fees are, I'd imagine this could work quite well. It is unclear how well it will scale, but if they can keep their costs low enough it may not need to.