Welcome to FT. In a nutshell it is their program and they can do what they want with it. When you signed up you agreed to their terms (
http://hhonors1.hilton.com/en_US/hh/terms.do) including the following:
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# Hilton HHonors Worldwide, LLC, reserves the right to add, modify, delete or otherwise change any of the rules, procedures, conditions, benefits, Rewards or Reward levels pertaining to the Program at its sole discretion, with or without notice, even though changes may affect the value of points or Reward Certificates or Confirmations already accumulated. This means that Hilton HHonors Worldwide, LLC, may make changes that affect, but are not limited to, Hilton Family hotels, travel partner affiliations, rules for earning HHonors points, Reward redemption levels, rules and procedures for the use of Rewards, continued availability of awards, Reward types, and the features of special offers.
I don't think anyone likes the devaluation, but the only thing you can really do is vote with your wallet.