That may be true in some cases but you'd be very surprised at the amount of discretion and leeway available in case they actually want someone and a recruiter contacting you is a sign that the hook is baited and that you should negotiate to your best abilities.
Revealing your current salary is dumb because (1) it blows your negotiation position to kingdom come and (2) it tells them something they can use against other candidates as well that are better at negotiations than you.
If you want to believe that all salaries are equal within narrowly defined bands ask yourself this: who benefits from such an arrangement and what can I do about it?