Because as we all know the laws of economics are fair and just. Truly a business man who strictly follows the laws of economics would always compensate people fairly for their value delivered to the company.
In all seriousness, how does this make economic sense? The company is incentivizing that people move to areas with a high cost of living, but why exactly? What economic benefit does that bring to the company?
The only reason that's ever been explained to me is that the cost of competing for talent is higher in those areas than others. However someone's location is a poor indicator for talent potential, so why would you compete in high cost areas?