Although Yahoo is definitely more constrained then Pinterest in terms of their license language, they actually do have a similar indemnity clause:
"You agree to indemnify and hold Yahoo! and its subsidiaries, affiliates, officers, agents, employees, partners and licensors harmless from any claim or demand, including reasonable attorneys' fees, made by any third party due to or arising out of Content you submit, post, transmit, modify or otherwise make available through the Yahoo! Services, your use of the Yahoo! Services, your connection to the Yahoo! Services, your violation of the TOS, or your violation of any rights of another."
You're describing the concept of "consideration" in contract law -- i.e., that each party to an agreement needs to exchange something of value in order for a valid contract to be formed. However, one doesn't need to pay money for something in order for there to be an exchange of value.
Here, the copyright holder (Mark) has clearly given you something of value -- a copy of the text of his book. In exchange, you give the him something of value, as well - your a promise to abide by the terms of the GNU FDL (as referenced on the book's copyright page). Your promise has value to Mark -- if it didn't, he (presumably) would have simply released his work into the public domain, rather than under a GNU license.
The enforceability of open source licenses turns on this issue. The implication of your statement is that an open source license is unenforceable whenever a work is distributed for free (and obviously, this kind of distribution happens all the time). While there were some in the legal community who took this view in the past, the courts have rejected it. Check out the 2008 Federal Circuit decision in Jacobsen v. Katzer (http://www.cafc.uscourts.gov/opinions/08-1001.pdf), which turned on the enforceability of the open source Artistic License. Some of the more salient quotes:
- "Traditionally, copyright owners sold their copyrighted material in exchange for money. The lack of money changing hands in open source licensing should not be presumed to mean that there is no economic consideration, however. There are substantial benefits, including economic benefits, to the creation and distribution of copyrighted works under public licenses that range far beyond traditional license royalties."
- "The choice to exact consideration in the form of compliance with the open source requirements of disclosure and explanation of changes, rather than as a dollar-denominated fee, is entitled to no less legal recognition."