I'm not a lawyer. You should listen to a real life lawyer's advice when you think somebody may have done something illegal, OP.
Job interviews are all at once impenetrably tricky and deceptively simple. It's always so hard to know what you should always do, and never do, and where it's a gray area because even HR professionals have trouble agreeing what's a standard hiring practice for even a few years at a time, and even then you'll be privy to any number of second-hand stories that completely invalidate whatever some expert you've never of heard wrote up on the internet.
It's why I treat interviews like first dates. Obviously looking and sounding good are important, but remember that candor always has a point of diminishing returns for both sides. This particular instance could rub up on legal questions (so do what smart lawyers on this thread have mentioned already and consult a real one in your state to actually know the score) but even if it isn't, sometimes it's helpful to ask why people are interested in something, or perhaps feign some misunderstanding to get a better angle on their agenda. People acting with less than good faith typically aren't very transparent and eager to answer questions about the nature of their own inquiries. If you're sitting down with a date, much less an employer, and they are super cagey about why they want to know especially personal information, it might lead a reasonable person to becoming at least a little suspicious.
Sounds like a bum deal regardless, OP. Grab a drink, dust yourself off, and find you another interview. Good luck.
