A government promise that they'll only do lawful things is not reassuring at all:

1. We've seen government lawyers write memos explaining why such-and-such obviously illegal act is legal (see: torture memo). Until challenged, this is basically law.

2. We've seen government change the law to make whatever they want legal (see: patriot act)

3. We've seen courts just interpret laws to make things legal

A contractor doesn't realistically have the power to push back against any of these avenues if they agree to allow anything legal.

(At the risk of triggering Godwin's Law, remember that for the most part the Holocaust was entirely legal - the Nazi's established the necessary authorization. Just to illustrate that when it comes to certain government crimes, the law alone is an insufficient shield.)

This is it exactly.