Rational v Irrationonal

Excellent question and point. 1) I see this as an example where we have become lazy with the English language—like taking a linguistic shortcut. "Irrational" has colloquially become to mean "believing or behaving in a way that with which I disagree." 2) This can also be due partly to the fact that the term "rationality" is used in multiple disciplines, most of which use the term in a slightly different way. 3) Sometimes we refer to an action or behavior as "irrational," when it would be more accurate to call the system of beliefs "irrational" that led to the actions. We can look at the your example considering this last point.

Rationality refers to both actions and beliefs, and the reasons for both. Flying planes into buildings, given the radical jihadist framework, is not an irrational action (one we strongly disagree with, yes, but not irrational). However, the beliefs that a radical jihadist holds could easily be argued to be irrational, as they are more likely to be supported by "faith," psychological manipulation, and other feelings rather than reasons.