Similarly, I never post anything on Facebook (or the Internet at large) that I wouldn't feel comfortable seeing on the front page of the newspaper. But the moment anyone asks for any account password, I'm done talking to them.
That said, companies have been requiring that applicants submit credit reports for a long time, so the idea that there's a bright line between what's "private" and what's "public" when it comes to employment isn't quite right.
I thought this was a really nice way of letting your employees have a choice even though in this particular case they probably could have forced everybody to do it. Now you have companies which do not really need to have this personal information asking employees for it. Here's the thing though. If my employees had lots of nasty things to say about my company then my reaction wouldn't be to stop them by monitoring everything they do. Wouldn't you want to try and fix the problem?
I once left an interview because the NDA they wanted me to sign gave them rights to all of my work for the next 6 months, and the right to audit me at any day or time by searching my house!
I told the HR person I wasn't willing to sign that, and why, that it would prevent me working anywhere if I didn't get that particular job, and she reacted as if I was being a prima donna.