[Carry_Okie]

I suspect that it might be legally questionable to quarantine someone who has tested negative, thereby prohibiting him or her from traveling to another state.

I suspect that depends upon the statute. There is precedent for instituting quarantine upon the basis of contact.

[independentmind]

Maybe— I am not attorney. But historically, interstate matters (which I assume to include interstate travel by US citizens) have usually been considered the domain of the federal government.