Why does this statement about intrastate aviation in Alaska appear in airworthiness directives?
It's there because the law requires it. 49 USC 40113(f):


  (f)Application of Certain Regulations to Alaska.—
  In amending title 14, Code of Federal Regulations, in a manner affecting intrastate aviation in Alaska, the Administrator of the Federal Aviation Administration shall consider the extent to which Alaska is not served by transportation modes other than aviation, and shall establish such regulatory distinctions as the Administrator considers appropriate.


Alaska relies on aviation to reach certain communities, and so Congress requires the FAA in its rulemaking process to consider whether aviation rules should be different for Alaska based on whatever it considers to be "appropriate." The cost/benefit ratio for a safety regulation could be different for Alaska—grounding certain aircraft may cut off some areas entirely—, so the FAA is required to at least think about the effects of their decisions on the state. 

The above assertion is a statement that they've considered Alaska when issuing the airworthiness directive and decided it should apply the same there as everywhere else.