They can't hack but if they can see anything from just going to it then they can. I wouldn't be surprised if some don't do that.
Employers are looking for character on FaceBook. They will decide if you are trustworthy, reliable, etc. from anything they might see on your FaceBook.
I think it should not be allowed. They are already asking for cover letters, Resumes, references, background check and credit check. You would think that would be enough. It is just too much power employers are having over you.
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them