A work authorization (EAD) applies to working in the US though. If the company is not located in the US and it is not liable for taxation to the US, I don't see why it's any of the business of the US if you're not violating US employment law.
Edit, to present a scenario: suppose my fiance owns properties in South Africa, which he rents out as passive income. Is he expected to sell these properties in order to qualify for a work auth? How does this differ from working for a non-US country while paying taxes to the country of origin, all the while bringing wealth into the US to add to the economy?
I'm neither an economist or a lawyer but I am struggling to see the drawback to make this illegal. You're not working in the US illegally and therefore withholding from the IRS. There's not even any way to report these earnings to the IRS because it's literally none of their business.
If anyone can clarify this, I'd appreciate it.