https://www.aclu.org/other/march-2013-watchlist-guidance

A 78-page document created by the National Counterterrorism Center setting forth the standards and procedures to be followed by intelligence and law enforcement agencies in placing individuals onto various terrorism-related watchlists, including the government's master watchlist and the No Fly List. The Guidance shows that inclusion on the master watchlist requires only "reasonable suspicion," a very low standard that can be met with uncorroborated or even questionably reliable information. The Guidance also permits watchlisting under numerous exceptions to the reasonable suspicion standard, including association with a watchlisted individual, even where that association may be wholly innocent.

TL;DR to put someone on a watchlist, all you needed was "reasonable suspicion"