My understanding is they can withhold $$$$ but any withholdings ( TAX) must then be paid to the US GOVT for and they will have to provide you with a W2 by jan 31st of the following year, that states your income and the TAX withheld from you for the previous year. The W2 you send in with your tax ( IE 1040 form ) is just proof that the company has sent that money to the US GOVT not what you earned.
eg, if based in HKG you do not send a w2 to the IRS when you file your taxes because all money is foreign sourced and you work for a foreign company. There has been no company withholding. What you have earned overseas is your responsibility as an American to report to the IRS and to file 1040.
CX USAB has to pay quarterly to the US GOVT. That money should be there as the company is supposed to be registered in the USA with a Tax ID Number. If they do not pay the US TReasury, the IRS will not allow you the ability to get your moneyback in the form of a tax return. (if you qualify) IF in fact this is the case, they ( CX USAB ) are in a very serious legal position. Many employers in the USA have been caught spendiing the employees withholdings, which is a serious crime.