It's just that the post you used in your question was a perfect example of that courtesy I believe is DUE many of the sites, authors and publishing houses whose works we use for educational and discussion purposes here.
It's not so much a link to "buy BOOKS at Amazon", it's a link to find Barbara Olsen's book if you're interested in reading more.
I think there's a substantive distinction between touting Amazon and touting a particular book or author -- and the subject of the thread at hand -- which one can find at Amazon.