It's much less paperwork and trouble hiring somebody who has a U.S. work permit when you're a U.S. company. No worrying about e.g. Brazil's employment or tax laws. That's basically it. Boring and unfortunate, but true.

It's not so much the U.S. employment issues as it is the international/cross-border financial transactions compliance issues. Cross-border payments are generally subject to significantly greater (tax) compliance requirements and the penalties for non-compliance are generally steeper than they are for purely domestic compliance failures.

When you have int'l employees, you have all that plus the additional headache of figuring out which nation's employment laws apply to your employee, and when, and in what proportion. Thus, a US employer might have to pay FICA/Social Security for a foreign employee, or it may have to pay the foreign nation's simlar such employment/payroll taxes. The fees for the legal/tax advice could easily be a multiple of the foreign employee's salary, as could the penalties for non-compliance.