If a US company operates in a different country as well, it has to abide by the laws of that country, or leave it. For example, Adobe's acquisition of Figma was blocked by the UK and EU regulators, despite them both being US companies headquartered in San Francisco. They could have chosen to leave the UK and EU markets entirely, in which case their merger would have been able to proceed, but they wouldn't have been able to sell anything to UK/EU citizens.