Interviewers (at least in the US) are NOT allowed to ask you for your:
* marital status
* religion
* age
* sexual orientation
- all of which are available in your facebook profile (provided you filled in that information).
So for that reason alone, it makes it illegal for them to access your Facebook profile.
Besides, giving out your passwords is not exactly a bright idea, especially since I know that many people are extremely lazy about inventing new passwords and tend to use the same password for several websites, which means you've just opened up a bunch of other places where that person could potentially log on as you. Do you really want this interviewer to take your identity? In theory he could log on as you and post some inappropriate things which would then tarnish your reputation.