Exactly. If companies don't adjust existing salaries to reflect that, then they are basically telling their employees to quit because the best way to get a "raise" is to switch companies.
But of course they won't, because salary information isn't available, so it is difficult for employees to make these decisions.
In other words, companies can systematically distort the free market in labour by withholding information about the true salaries it offers from its employees.
The door doesn't swing both ways though - people don't get a salary cut when supply later exceeds demand. If you wanted a salary system that did respond to job market fluctuations, you'd have to open up to the possibility of getting cut pay.
