It's not fair for an employer to look in your private life; any more than it is fair for the potential employee to look at confidential company data. How would you like that? The company is hiding information, thereby cheating you into a job.
Information in negotiations works two ways: oftentimes, it's prudent to release a bit of information as a signaling device, because it shows that you're confident the information will work in your favor.
Can I be any more clear than that?
It stops being private when you publish it to the world. It doesn't stop being personal. Some public, personal details cannot lawfully be considered in a hiring decision. But most can. It is your responsibility to guard your privacy. It isn't your prospective employer's.
Every time I interview for a job, I try to find out information from the company, particularly information they don't want released. Maybe I'm weird, but I get burned less because of it.