there’s a bit of context to all this, especially with the US, too. I’d be saying the same thing if it was a Russian or Chinese subject by the way, but the US are notable non-signatories of the Rome Statute, which means that they don’t recognise the legitimacy of the International Criminal Court, dealing with things like war crimes, acts of genocide etc.
So notwithstanding the historical context of its foreign policy, regime installations, black-ops etc, there’s a clear mandate of ‘FU’ towards arbitrating these sorts of things within the international community. Think its also relevant that the UK and US are massive buddies, and have been cosying up to each other even more recently. Were it, for example, an Iranian or Russian diplomat that had been detained or whatever, I think there’d be some cause for eyebrow raising simply due to the geopolitical context.