They are ignorant because they simply buy in to the common legal opinion, which is itself a byproduct of previous legal opinions that have been misrepresented as to intent and scope.
The Wong Kim Ark decision is not wrong if looked at in the right (minimalist) light. The idea that it justifies anchor babies and birth tourists is exactly wrong. Wong Kim Ark never went that far.
But that is the common opinion. It is ignorant, and it is wrong, and all 535 members who hold it are also wrong. If the nine justices of the Supreme court hold that opinion, they too are wrong. It is objectively wrong, but thanks to misinterpreted precedent, a large majority believe this wrong thing to be true.
The Wong Kim Ark ruling was meant to be widely interpreted not narrowly interpreted. At any time since 1898 Congress could pass legislation excluding anchor babies from qualifying as Citizens of the United States At Birth.
United States v. Wong Kim Ark (1898)
For lurkers here who may be unfamiliar with the particulars:
Facts: Wong Kim Ark was born in 1873 in the city of San Francisco in California. His parents were both Chinese immigrants and remained subjects of the Chinese emperor while they lived in the United States. Ever since he was born, Wong Kim Ark lived in California. Congress passed the Chinese Exclusion Act in 1882, which denied citizenship to any Chinese immigrants and did not allow any new immigrant laborers to come from China until 1892. In 1890, Wong Kim Ark’s parents returned to China. He visited them that same year, but he came back to San Francisco, and he was then recognized as a “native-born citizen” by the U.S. customs officials. In 1894, when he was 21 years old, he went back to China to visit his parents again. In 1895, he attempted to re-enter the United States, but U.S. customs officials denied his entry, claiming this time that he was not a U.S. citizen.
Issues: Does a child born in the United States to parents of Chinese descent become a U.S. citizen by birth, according to the Fourteenth Amendment to the Constitution?
Holding: Yes. The Chinese Exclusion Act was passed 14 years after the Fourteenth Amendment, so it cannot possibly control the meaning of the amendment. Justice Horace Gray wrote the opinion of the Supreme Court, which stated that the Chinese Exclusion Act “must be construed and executed in subordination” to the Fourteenth Amendment. The court held that the government cannot deny citizenship to ANYONE born within the United States, including Wong Kim Ark. Furthermore, if he was a citizen, then the Chinese Exclusion Act could not apply to him. Wong Kim Ark’s parents, in particular, were not engaged in any diplomatic or official capacity in the United States at any time.