All Cabinet people resign. It's up to the President to accept their resignations.
In any case, I expected Ridge to resign. He can make a load of money in the private sector as an expert consultant in the field of national security.
Lowering the threat level so that you can raise it again makes you an expert?
No, lowering it based on intelligence, and raising it likewise does. There is a heck of lot that Ridge could do, ranging from becomming a lobbyist to opening a law practice, to consulting, to managing a company.