[justanotherfreeper]

Just out of curiosity, will they have to pay US income taxes, like most of us will, on this already taxed money?

My understanding is it depends on whether they have to pay income tax as foreign nationals (some foreign nationals pay US income tax) If they pay US income tax yes, if not no (unless Mexico taxes the income, which I don't know.)

[Regulator]

You both are correct. Depending on the visa type, tax treaty with the country of residence, and IRS "substantial presence" test, an alien working in the U.S. may not be liable for either income taxes or any of the other taxes. See the IRS page here: IRS Tax Treaty Page

Note that in the case of people working here who are exempt, that it is another incentive to employ foreigners. Because all taxes are booked as a cost to employ someone; it's just a shell game to call them a tax that the employee pays.