1 An employer cant dictate who files tax returns. That is a matter for legislation, and in this case the IRS
2 You are only taxed on your worldwide income if you are tax resident - in the US that means a citizen, a green card holder or if you spend over 120 days a year in the US or its territories (but the calculation is based over 4 years on a sliding scale)
3 Many countries, eg the UK, have special rules for seamen or people working on ships and some have extended this to other forms of transportation
4 The US will tax you on income earned in the US. For example if you are paid for consultative work by a US company. This is regardless of residency or time in the country. If there is a double taxation treaty with your country of residence, you can claim relief from tax on that amount in your own country but only if the same money would be taxed in your country and to the amount of tax you would have paid
So your employer is irrelevant. If you are not US tax resident you dont pay tax UNLESS the IRS has changed their interpretation of the tax code or the tax code has been changed by Congress. It would be sensible to seek advice whether this has happened from a US CPA as opposed to taking advice, including mine, on Pprune