Believe it or not, immigration authorities (like the US Immigration and Customs Enforcement agency) have the power to enforce immigration laws.
The author isn't American.
Edit - wait until y'all find out other countries also have borders and laws...
Apparently they have the power to murder and kidnap American citizens too, or violate their rights if they happen to freely speak or assemble in ways they don't like.
Which immigration laws are they enforcing in this case? And are you also going to suggest that the Constitution does not protect foreign nationals inside the US?