The reason that this sort of legal action is necessary is because the kind of people who are being asked this aren't the kind of people who can walk into any company in the valley and get another job. In those situations, the employee doesn't have any cards to play.
I understand your point completely, and I agree with you. It still doesn't change the fact that when you give up your Facebook account information, you are not just surrendering up your personal information, you are giving up the personal information of everyone in your network that has chosen to share with you. It's a breach of trust with that network.