This not an 'immigrant' thing, it is a H1B requirement. When I initially began working in the US under Trad NAFTA there was no such requirement, but when I changed to a permanent track the job had to be posted. It is part of the process to prove that there is not a qualified person already available to fill the position. The whole underlying proposition of H1B is there is a shortage, disclosing the salary is just part of the process of proving it, which falls to the company making the petition. Really, I never gave it a second thought.