The UK fined it £520,000 but has no way to enforce it.
Despite the memes, the UK is not the 51st state, which is both why it was able to make the fine (1st Amendment doesn't have jurisdiction over the UK) and why it's not able to enforce it (the UK doesn't have jurisdiction over 4chan).
It's a matter of enforcement. And who would want to enforce 1st amendment?
The entirety of the US constitution carries zero weight to the court system of the nation that the founders of those United States, ahem, engaged in unlawful violent treason against, with the assistance of foreign powers (France, then Spain and the Dutch Republic). :P