Hi Maha, you write <>
That is OK for author as well as readers, because the open work remains open. Nobody needs to buy the book to do the course, the course still is open and anybody else could also make a book of it and try to sell it. (some open source software does also exist as commercial software. But CC is not a license used for software!)
The authors did CC their work and did not want any money, that is not changed. If they do want money they should not CC_BY it but keep it copyrighted and closed.
on http://creativecommons.org/ is links to discussions on the subject. (http://wiki.creativecommons.org/Considerations_for_licensors_and_licensees#Considerations_for_licensors)