// since when have US citizens (presumably in the US) been subject to UK laws //
My answer was in regard to the wider issue of international law and crimes against humanity, of which Begum now finds herself embroiled.
The UK government removed her British citizenship and the Netherlands is a non-starter. The motherland of her parents will not recognize her. In fact, Bangladesh have threatened her with the death penalty if she ventures into its territory.
Thus Begum is 'stateless' and her detractors seem unable to provide any evidence that she's committed terrorist crimes, other than to be a member of a terrorist organisation.
In Finland it is not against the law to be a member of a terrorist group. The USA administration turned a blind eye when some of its civilians were openly supporting the IRA. Therefore I see no reason why the UK should deny her British citizenship on the basis she joined a TO in her adolescence.
Begum's situation does not sit well with me!