If a quick google of a candidate showed a blog post where he's railing about his current job "fuck this employer, I'm going to trash the database on my last day and put a lot of bugs in the source code", then I WANT TO KNOW THAT.
A law that says that I can't know that is BS, and I'll find a way to comply with the letter of the law while still getting the information I need.
At least with the idiot, you can keep an eye on him.
For example, what if an HR employee can see from their public facebook page that they appear to be a fan of LGBT organizations and are interested in same sex people?
This exposes the HR person to information that is illegal to gather in an interview and illegal to use as the basis for job discrimination. You can also use marital status or interest in having kids if you prefer... Doing background checks that are not strictly work related is a huge issue. Your example is work related, so I empathize with your desire to know about it. But looking into non-employment behaviour opens up a huge can of worms... For employees and employers.
Your sex and race are instantly apparent, age and many disabilities can be guessed, national origin can be figured from an accent, and marital status can be seen by looking for a ring. Even religion can be discerned in some cases, for certain orthodox members.
I agree that employers being able to easily see an employee's sexual orientation might not be a step in the right direction, but I don't think it's opening up a huge can of worms. It's an already opened can of worms.