Er - I think most people know that calling someone a name related to fruit is somewhat different to a name related to sewage.
I respectfully suggest that if you were to call, say, an African or an Asian race "Sewage" - short for for Sewage Farm - a direct equivalent to calling someone a septic tank - on the basis of some fanciful connection to a "rhyme" you would find yourself correctly and rightly buried in all kinds of trouble - for being nastily racist. So why/how is it OK to call Americans that? Is it becoz they is white?
Does not compute.
Anyway, racism apart septic is a thoroughly nasty and offensive term, you wouldn't dare to use it to an American's face would you? It is grossly unpleasant and thus not acceptable by any normal standards of manners or decency.