I can't tell you how widespread the practice is, but I can tell you a story of the exact situation you describe happening at a top NYC FM station back in the early 1980s.
A friend of mine, who had prior experience in the NY radio market, had taken a PM drive job at a market leading station on the West Coast and had been there a few years.
One day I get a phone call that he has been offered a mid-day gig at one of the top rated stations in NYC and accepted. Both he and his wife give notice on their jobs, and he asks me to keep my eyes open for a house they can buy in the area etc. etc.
All the plans have been rolling for a couple of weeks, when I am scanning the NY Times help wanted section and I see the job advertised. After a panic call to the West Coast he assures me that the NYC station had warned him of the ad, and says that its purpose was to meet EEOC requirements.
While I was delighted that the station wasn't going behind his back, and wasn't going to leave him stranded on the West Coast without a job, I also thought of all the poor radio types who would spend that week assembling air checks, resumes, etc etc. when their effort was all wasted and in vain, since the job was filled long before that ad was ever placed. If that late applicant was out of work, there was distraction from an effort that could actually find a job, and there was the expense, and misplaced hope, that responding to the ad caused.
Something is very wrong about that system, and it's too bad that all these years later it hasn't been fixed. Since for most people, the number of radio formats they fit is small, and the number of stations not that many, the best thing to do is to keep your name and availability on file at all the stations where you would like to work. And when you see an ad, you really have no choice but to respond, but don't get your hopes up.
In the case I mentioned, the person at the NY station who made the decision on hiring was very familiar with my friend's work when he was at a competing station, and my friend had "kept in touch" while he was on the West Coast. It was the legal requirement to run the ad that caused the problem.
By the way, that was the only top level on-air radio job I ever remember seeing in the NY Times, the station really didn't want the flood of responses from radio types around the country it would have gotten with ads in Broadcasting magazine or Radio and Records, or the other usual media. It probably hoped that the ad would go unnoticed by radio job seekers, and yet was in the "newspaper" of record for legal purposes.
