The laws on this vary by state[1] and are sometimes broad (e.g. Connecticut allows truthful statement of any facts) but looks like about 20% explicitly allow disclosure of pay.
[1] http://www.nolo.com/legal-encyclopedia/free-books/employee-r...
Because 1. As a job searcher you always use your previous previous job as reference and 2. No potential recruiter would be stupid enough to call up the current employer. I wouldn't be surprised if this actually happen though.
Not every job search is done without the knowledge of the current employer. Just a few examples of when that may not be true:
* moving across the country because of a spouse's relocation
* contract employee approaching the end of the contract period
* full-timer looking to switch to part-time
* switching careers
* volunteering for an "early retirement" deal at a company that's downsizing
* difficult to replace role (e.g. it's not uncommon for CEOs of large companies to announce their intention to leave a year or more out)
I do agree that recruiters (or anyone) should never contact your current employer without your permission but it's incorrect to assume all job searches are carried out surreptitiously.