Thrown Off a United Airlines Flight for Taking Pictures 13 years ago
Meh. Passenger demonstrates willingness to a) revisit the incident later and b) use the word 'terrorist' on the plane.
They don't have to actually think he'll blow up the plane; it only takes one closet nut case to overhear this and cause a ruckus mid flight. In a small confined space at 30k feet that's a very real safety issue. Consider that an irrational fear of flying isn't that uncommon, and this is a route where there isn't much of an alternative.
Finally, they're under no obligation to tell a passenger the real reason for booting him off the plane; if it was the above, doing so could only make it worse for themselves.