The problem with supporting legal immigration only is that Congress has been unable to pass common sense immigration reform over the past decades, because there's not enough political backbone. (And also the Republican Party has made it increasingly toxic)
Illegal immigration = the difference between legal immigration supply and demand for immigration (both external and domestic)
And the rioting / nativist arguments are political theater to score points.
It's a fact that most illegal immigrants don't commit crimes or riot, in the same way that most people in general don't. But distorting frequency and total count has long been a popular pathos argument.
Similarly, enculturation and assimilation is a broader topic than a flag. And more over, the US explicitly and unambiguously grants freedom of expression as a right (all persons in America, not just citizens). So saying "I hate this country; I support ____" is and has always been an option as an American.