The money is NOT refunded. This is an international personal tax levied on every single individual world wide,who are NOT American and are not working in America, but because they are working for an American company online, they are pay a flat tax (non refundable) between 10% and 30%. This is a called a tax treaty.

How does it address tax evasion when the only people who are supposed to pay American tax are American people.

No other country in the world does this. I think you should read more about this.

https://www.irs.gov/businesses/international-businesses/united-states-income-tax-treaties-a-to-z