[ddlsmurf]

There is a due process to catch an ally's civilian, that's called an extradition, that process is important.

[bencollier49]

The parties involved probably judged correctly that if they attempted to extradite him, he would be the subject of a prolonged media campaign against the government in the UK to keep him here.

What I don't understand is why the FBI didn't just hand the evidence to the NCA in the UK and have them arrest him.

[ddlsmurf]

Well the government shouldn't modulate how it executes the law based on the optics or media impact. And as you said, this move shows that they didn't think the UK would agree with their evidence, which is far more reason for this snatching to be worrying.