Saying politicians direct government scientists is like saying CEOs direct programmers. There's a connection, but it's very, very indirect. Don't confuse public policy statements with the work done by scientists.

I'm aware of incidents like [link|http://www.nytimes.com/2006/01/29/science/earth/29climate.html?ei=5088&en=28e236da0977ee7f&ex=1296190800&pagewanted=all|Hansen's]. And we know that scientists have biases just like everyone else. But good scientists aren't directed by politicians to find a particular result to support a particular policy. They're directed to investigate a problem and accurately report the results. Hansen's case, as I understand it, was a conflict between his appropriate role in reporting research results versus the perception that he was making policy statements for the government (something that isn't his role).

For example, the National Science Foundation funds [link|http://www.nsf.gov/awardsearch/progSearch.do?SearchType=progSearch&page=2&QueryText=&ProgOrganization=&ProgOfficer=&ProgEleCode=1530&BooleanElement=true&ProgRefCode=&BooleanRef=true&ProgProgram=&ProgFoaCode=&RestrictActive=on&Search=Search#results|climate research] even though the results may contradict the Administration's positions. It's similar in government labs. (Of course, politicians determine (in the broadest sense) what research gets funded. But they don't direct research outcomes.)

Cheers,
Scott.