It's usally defended in some lame 'seperation of powers' arguement.
http://www.usdoj.gov/oip/procereq.htm#entities
It says in part (very near the beginning) that:
Entities Subject to the FOIA
Agencies within the executive branch of the federal government, including the Executive Office of the President and independent regulatory agencies, are subject to the FOIA. However, the FOIA does not apply to entities that "are neither chartered by the federal government [n]or controlled by it."
Thus, it is settled that state governments, municipal corporations, the courts, Congress, and private citizens are not subject to the FOIA. Nor does the FOIA apply to a presidential transition team. (footnotes removed; emphasis added)