This is a flawed philosophy. As a manager I have had a few critical pulling in higher salaries than my own. If you are a superviser who is artificially holding an employee back from reaching their full potential (seeking compensation that matches the demand person's talent in the open market) than you are doing your company (yourself) and that employee a major disservice.
Just because you are a manager doesn't mean you can do what your employee does (and vice versa). Artificially holding a sub-employee back is a sign of fear for one's own ineptitude.
EDIT: My main responsibility is to keep the team happy and productive. If this developer would leave the team our productivity would drop and we wouldn't achieve our goals we set for ourselves this year. It would have a direct impact on my bonus as well and it is in my highest interest to keep great people at every possible cost. If I think I don't earn enough then I will have that chat with my line manager, but I will never try to destroy my team based on some personal ego crap.
Which is better for the company? (A) Leader acts in such a way to make him or herself 150% effective, or (B) Leader acts in such a way as to make all of his or her direct reports 115% effective?
If a leader can't make their team more effective, then why have them in leadership?
(Edit: Essentially, what dustinmoris said!)
It is basically just a question of market forces. If a particular position is sufficiently specialised when the supervisor role is more generalised then you might expect to pay the specialty more than the generalised role - even though the generalised role is more senior.