The charge is to upgrade the status for 90 days as a trial period. They are asking for money because the person requesting the upgrade needs to show good faith that they will fly the subsequent miles required in the next 90 days.
They are getting the benefits of the next upgrade for 90 days in exchange for paying money.
When we pay USAIR for their services, they can pay their employees. Then their employees have money to buy things which helps others' business.
In essence USAIR is saying that they will allow it for a fee. They do not have to allow it all.