I’m sure there’s more to the story than this.
Apparently the parents were originally from South Africa, and the kid holds a South African passport.
To enter the USA with a South African passport, one is required to additionally possess a visa for entry. The same is not required of a British passport holder.
The parents failed to obtain the appropriate passport, and instead attempted the kid’s entry into the United States using a wrong passport without the adequate visa.
I have extensive experience with sort of thing. There is nothing more to it than berserk INS agents. (They don't call it the INS any longer, however.) It is very common for people to be denied visas under that excuse, especially if the applicant has an education and is young.
This is pretty stupid in view of conditions on our Southern border; but, most of what our government does is just plain stupid.
"Never ascribe to malice that which is adequately explained by incompetence"