From Craig Nelsen:
The inference that "We are a nation of immigrants and, therefore, we must not limit immigration" is a classic example of CIRCULAR ARGUMENT.
What it says is this: Because we are a nation of immigrants, we have to allow for massive immigration which, in turn, makes us a nation of immigrants. Hence its circularity.
Circular arguments are invalid in the logical sense by virtue of HOW they are structured and not WHAT do they mean. They lead to a faulty (and, therefore, useless) reasoning in which the thesis (the very thing which is to be proved) is used as a premise in its proof.
The "nation of immigrants" drivel is rather idiotic and totally false. I don't know why people keep repeating it.
Because they are misguided, or they have an agenda.