For the same reason as anyone else: they were born in the US (or one of its territories), or at least one of their parents was a US citizen when they were born.
The way you ask the question sounds as if you think they shouldn't be, though frankly it's a mystery to me as to why.
According to the constitution, yes.
Yes. Persons born in Puerto Rico, the District of Columbia, Guam, and other US territories are US citizens.
The Fourteenth Amendment to the Constitution declared all persons born or naturalized in the United States to be U.S. citizens. It was adopted in 1868.
Civil Rights Act
Civil Rights Act
The 14th amendment deals with citizenship. It does not quite say that all persons born in US are citizens-- only those born in US and and subject to the jurisdiction thereof. I am not a lawyer, but it seems to me that foreign nationals would not automatically become citizens by this amendment if they happen to be born in the US.
The Amish got to America by ship. Back then, there was no such thing as an airplane.
The Amish are different from us because they live simply while we don't live simply.
The 14th amendment
Persons born in the Virgin Islands are U.S. citizens.
"The purpose of US border patrol is to insure that persons crossing the border into the US are person that should be allowed into the country. The border patrol checks to make sure that persons entering the country are citizens, have a passport, or have a VISA."
you would do it for free because thats what nice citizens do.