Organizations reserving the express right to unilaterally change the terms of their policies?...Is that legal?...

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Many critics have attacked the efficacy and legitimacy of privacy policies found on the Internet. Concerns exist about the effectiveness of industry-regulated privacy policies. For example, a 2000 FTC report Privacy Online: Fair Information Practices in the Electronic Marketplace found that while the vast majority of website surveyed had some manner of privacy disclosure, most did not meet the standard set in the FTC Principles. In addition, many organizations reserve the express right to unilaterally change the terms of their policies. In June 2009 the EFF website TOSback began tracking such changes on 56 popular internet services, including monitoring the privacy policies of Amazon, Google and Facebook.[39]

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(39) Millis, Elinor, "EFF tracking policy changes at Google, Facebook and others," Cnet Digital News, June 2009. Cnet.com

http://en.wikipedia.org/wiki/Privacy_policy