Because they can easily learn so much about applicant bad habits, right?
Or maybe not. Let us never forget that one is not supposed to inquire about a job applicant's marital status, age or various other characteristics. In fact, if one does it can lead to quite amusing legal consequences for the interviewer's organisation.
Read I hereby (fictionally) resign to see how requests to see facebook pages could backfire.
I just saw this on today's O'Reilly's Four Short Links, part of a feed for programmers and other geeks.