Any individual hired by a US company in which the end service (as opposed to a product) is within the United States, should have the same legal requirements as a employee within the borders of the US.
These workers would be required to have a tax payer ID and companies would be required withhold income taxes, medicaid and social security just as they would regular US citizen employees living within the US.
Taxes would be non-refundable.
A few examples of services with endpoints within the US would be:
Customer Service
Data Entry
Software Development
Radiology