Fear, loathing, suspicion and monitoring of civil rights movements.
All it took was the thought that foreigners were influencing Americans.
That's all it took to make the massive surveillance "legal".
Of course, massive surveillance means more than just surveillance:
* Main Justice, by Jim McGee and Brian Duffy, 1996, ISBN 0-684-81135-9
*
* The FBI had been spying on members of the civil rights movement
* to discredit Martin Luther King and destroy the civil rights
* movement, government files showed. There had been burglaries
* and illegal wiretapping on a grand scale.
Even after FISA legislation, with its strict "minimization" requirement,
CISPES & Co. happened.
Sometimes their suspicion of terrorist/foreign agent activity is laughable.
* "Above the Law", by David Burn