mickeyjoebill, prior to 9-11, the chance of them boarding four different flights and successfully blowing tnemselves up in the air would be pretty high. Even after 9-11, as Russia found out, such terrorism can succeed, particularly when screening is poor or compromised.
These days, terrorists would rate the chance of successfully hijacking a plane as extremely low, while the risk of apprehension beforehand is rather high. So they migrate to easier targets.
On a somewhat related note, the U.S. Department of Homeland Security is abandoning the stay in your seat rule for flights arriving and departing Washington's Ronald Reagan National Airport. (The rule required passengers to stay seated for the first 30 minutes of any flight departing KDCA, and the last 30 minutes of any arriving flight . If somebody got out of their seat, then the plane could be diverted to an alternate airport. And on flights between National and LaGuardia, passengers were required to remain in their seats the entire flight.)
This rule was among the most stupid ever imposed, as passengers flying into nearby Dulles or Baltimore Washington International airports were under no similar restrictions, nor were passengers on planes overflying the area required to take their seats when the airplane was in the vicinity of Washington DC.