Their wealth is proportionate to the amount of value they deliver in the marketplace.
So what? I don't understand why this is an issue.
Inequality is bad for economy. A person that owns 100,000x of average doesn't spend 100,000x more. They have to invest the money. But there's nothing to invest in given that poor people don't have much money to buy stuff and rich people are all trying to invest rather than consume. So in the end they are forced to invest in dubious stuff like subprime debt and rely on government to pay the bill. Or to ride economic bubbles and hope to cash out right before they burst.
This is entirely conjecture as written by a philosophy professor.
A philosophy professor who writes about things like social contract theory, which is precisely the topic that attempts to explain what impact inequality has on a society. If anyone is in a position to explain why wealth inequality is a problem, it's T. M. Scanlon (or any other eminent philosopher).