Some American politicians - particularly the Trump administration, its allies and officials - have pushed back against what they regard as overreach in the regulation of US tech firms by the UK and EU.
A perceived impact of the Online Safety Act on free speech has been a particular concern, but other laws have also been the source of disagreement.
On 19 August, US Director of National Intelligence Tulsi Gabbard said the UK had withdrawn its controversial demand for a "backdoor" in an Apple data protection system - saying she worked with the President and Vice President to get the UK to abandon its plan.
Two days later, US Federal Trade Commission chairman Andrew Ferguson warned big tech firms they could be violating US law if they weakened privacy and data security requirements by complying with international laws such as the Online Safety Act.
"Foreign governments seeking to limit free expression or weaken data security in the United States might count on the fact that companies have an incentive to simplify their operations and legal compliance measures by applying uniform policies across jurisdictions," he said.
If 4chan does successfully fight the fine in the US courts, Ofcom may have other options.
"Enforcing against an offshore provider is tricky," Emma Drake, partner of online safety and privacy at law firm Bird and Bird, told the BBC.
"Ofcom can instead ask a court to order other services to disrupt a provider's UK business, such as requiring a service's removal from search results or blocking of UK payments.
"If Ofcom doesn't think this will be enough to prevent significant harm, it can even ask that ISPs be ordered to block UK access."