Giving John such an explanation for why his salary is set as it is and what he can do to get a raise is not a problem for an employer. It's about the best possible case for them really.
The real problem for companies is when the explanation comes down to "Sandy negotiated harder than you", "Sandy got a higher offer from a competitor so we had to match it to keep her", "we're paying you significantly under the market rate and are hoping you don't notice or aren't too bothered by it", etc.
The real problem for companies is when the explanation comes down to "Sandy negotiated harder than you", "Sandy got a higher offer from a competitor so we had to match it to keep her", "we're paying you significantly under the market rate and are hoping you don't notice or aren't too bothered by it", etc.