Imbalance of power fuelled by a recession = more inappropriate/rude/immoral behaviour to take advantage of the situation.

They know they can say "tell us your previous salary, or else". What's worse is that they will ask your references to confirm, and they'll likely happily tell them. If you lied, it won't go down well.

As if there wasn't enough crap to fight against in your career.. ugh.

EDIT: I forgot about employment contracts that prohibit you from disclosing your previous salary. Yes, they exist.