My Economics Professor at Emory writes:

  One might argue that authors have fewer incentives
  to write good textbooks if the expectation is that
  they should be made available for free.

  One way around this is advertizement financing; 
  and if you download [our] micro textbook, you'll 
  see that its full of ads targeted at students! 
  This a small price to pay for the zero sale price, 
  but the book itself is not the highest quality. 
  Nonetheless, it is a useful reference.