Thanks for spreading the word haha
The 100-mile border zone derives its authority from the Immigration and Nationality Act of 1952, specifically 8 U.S.C. § 1357(a)(3), which permits CBP agents to conduct warrantless searches and seizures “within a reasonable distance from any external boundary of the United States.”
That extends to intl airports as well. You also don't have a right to an attorney:
CBP takes the position that travelers do not have the right to an attorney during questioning.
This has nothing to do with CBP or the Immigration and Nationality Act of 1952. And even if it did, if they believe they don't have the right, and no one tells them otherwise...then they do.