Why can't you ask if someone is a US citizen? I thought certain jobs (e.g., DoD contractor needing security clearance) required citizenship? Or are you only allowed to ask about that in specific circumstances?
Where citizenship is a condition of employment by statute, that condition is usually exempted under anti-discrimination legislation. For example, my little brother is an aerospace engineer designing for the JSF project - "Are you a citizen?" was pretty much the first interview question.
From what I've read, you're apparently supposed to ask "do you have a security clearance" and "are you eligible to get a security clearance" -- i.e., ask the specific question you need answered rather than asking a more general question from which you can infer what you want to know.
A 5 second Google search will show you that there are plenty of jobs that overtly require US Citizenship. For example, all TSA screeners must be US Citizens.
This is kind of a diversion from the real discussion. It's unlikely that anyone reading this board can legally ask interviewees about their citizenship.
It's not illegal to ask a question which in answering, the applicant may offer such information.
In all of the examples you give, the "legal" question is askking something directly relevant to whether the applicant is able to perform a job. What someone does with their free time may give you some insight into how well they would do a job, but it's far less directly connected.
* Marital or family status (ask a working mother about this one if you want your blood to boil)
* Age, apart from legality to work
* Sex or, in most places, sexual orientation
* Citizenship, ancestry, race, or national origin, apart from legality to work
* Disabilities of any kind
* Home address
Most questions that would be reasonable in normal conversation are OK in a US interview, so long as they don't pertain directly or indirectly to "race, color, religion, sex, national origin, disability, or age".
You are not strictly limited to questions that directly pertain to a candidate's ability to do the job. In most private business settings, you can legally choose not to hire someone for any number of subjective reasons. Not staying up late to work on open source projects is certainly one of them.
On the other hand, if you have a strong preference for someone who codes in their spare time, and you care about not being a douche, you should mention that preference in your job requirements. It's not cool to waste people's time.
Correct. It is possible to be authorized to work in the United States without being a citizen. Asking about citizenship directly can be illegal because it can be a proxy for excluding recent immigrants who have green cards (permanent residency status) but not citizenship.
Illegal: Are you a U.S. citizen? Where were your parents born? What is your native language?
Legal: Are you authorized to work in the U.S.? What languages do you speak?
Illegal: Are you married? Do you have, or plan to have children? Have you ever been divorced? What do you do for child care?
Legal: Are you willing to relocate? Are you willing and able to put in the amount of overtime and/or travel the position requires?
Illegal: Are you a U.S. citizen? What is your religion? Are you practicing?
Legal: Does your religious affiliation, if any, interfere with your working in this position?
Illegal: How old are you?
Legal: Are you over 18?
The very first interview question I ask is "Tell me about yourself".