If seeing an applicant's Facebook profile is important to employers, then the market wage for an applicant who says "yes" is higher than that of an applicant who says "no". Based on that, allowing people to say "yes", even when they're doing so for lack of another good option, lets them make more money than they would otherwise make. Banning them from saying "yes" prevents them from capturing that higher wage.
A law that "protects" applicants from having the option to say "yes" lowers their income. Giving them the freedom to answer the question either way leaves them the choice between a higher income and the privacy of their Facebook account. It's not a happy choice, but if I valued the first more highly, I wouldn't like the government to force the second on me.