They are americans by nationality, nothing more.
If having an american passport makes you an american, then there shouldn't have been places named as 'chinatown', and references of hatred like 'spick' or 'blini' wouldn't have been there..both of which along with others refer to members of white race.
No matter how many of your generations have lived in UK, all it takes is to err on the road, and the first thing you'll hear is the word Paki alongwith some animal nomenclature.
A british passport wouldn't make you a briton. Nor an american passport an american.
The discrimination is and will be there.