Notice, however that it's not about _what_ to do, but _how_ to do it. And when it comes to to the how of human space flight, NASA has the experts, not Congress. Politicians rely on Engineers and Researchers to make informed decisions, it is therefore only a natural expectation on behalf of Congress that unreasonable, meaningless, or dangerous requests should be denied. If that doesn't happen the Administrator screwed up, it is that simple.

I think what you mean to say is congress approves the appointment of officials nominated by the executive branch and approves the budget for agencies. Agencies have no control over congress. The executive branch can dictate what an agency can do, within the boundaries of a law originally passed by the congress (though almost never a recent congress).