I agree and to add - the UK and Spain in particular have a long history of maintaining oppressive rule over unwilling subjects, and the US has a long history of at least notionally backing the self-determination of these subjects.

To start with the UK - the US fought for its own independence against the UK, the US supported decolonization of Africa and Asia against the UK, the US supported Egypt against the UK during the Suez Crisis, Americans supported the Catholics of Northern Ireland in their struggle for civil rights.

For Spain - to echo what you said, thousands of American volunteers and millions of American dollars supported the Republicans in Catalonia. Working backwards, forty years earlier the US took up the cause of "Cuba Libre". From the start of the 19th century the US pledged to support the new American republics against European conquest (even though the Monroe Doctrine didn't amount to much practically).

So sure, today Scotland and Catalonia probably aren't that bad off.

But tbf there's a reason why many Americans are inclined to trust the substantial separatist groups who clamor for liberty from British/Spanish rule, and why they are inclined to distrust the British and Spanish governments when they insist that no really, this time they really aren't oppressing anyone. It's like whatever the opposite of "the boy who cried wolf" is.