Demosthenes2:

Employers hire people ‘at will’ based upon market value. My company doesn’t need to pay those at equivalent positions the same as they pay me. They just don’t.

That's true to an extent. However, it is illegal under federal law to discriminate in pay based on certain protected classes, including race, religion, and sex. It is also illegal to retaliate. State laws provide additional protections.

Companies know this, so if they are smart, they have a policy that explains how they determine pay, and can demonstrate that the policy is non-discriminatory.