If the advertiser stipulates US Passport you can take it that it is a requirement of that particular operator... probably due to them working in sensitive areas and needing a security clearence that is much easier to obtain if the applicant is a US citizen... there is no legal requirement to be a US citizen.
It is possible for a company (in theory) to sponser an expat applicant and there are provisions within the immigration act to allow for this. Having said that, it is available only to recognized professions where positions are difficult to fill with local (US based) staff... AT THE PRESENT TIME this does not include pilots flying part 121 / 135 or part 91 operations (other than flying instructors).... if you are an experienced test pilot trained by the Empire Test Pilot school and familiar with advanced systems development you may well qualify.