Being fairly compensated and being adequately compensated are two different things. Sometimes you have to pay more than what is "fair" to keep people. Fair is tied to market forces, forces that assume people can/will move between jobs. To stop people from falling prey to market forces and leaving you need to pay them above the going rate, beyond fair.

An extreme example of this is where the fair/market rate is zero, or even below zero. People like interns on film/tv productions are paid nothing, sometimes for years. There are even examples of people paying for the opportunity to work without pay. If you want such people to remain loyal you have to pay them something, which would be more than the fair/market rate of nothing.

For an example of negative market rates in the medical field, see http://thetab.com/uk/uclan/2015/10/04/is-uclan-elitist-1193

"There has also recently been criticism of work experience placements in a private hospital being sold for £500 per week, which would help students improve their applications to study medicine at university."