I hate to sound so heartless, especially since I live in Israel, but... I don't see what's so "special" about this (FROM AN AIRLINE'S PERSPECTIVE, not from mine, so please don't lynch me), that an airline would be expected to allow people to reschedule/cancel for free due to the "situation". I mean, do you also expect an airline to allow you free changes to tickets to/from DTW, because "Detroit is a high-crime city", for example?
It's one thing when rockets are falling, chas veshalom - which are as unpredictable as they are unique (i.e. not a common crime/danger anywhere else in the world). But stabbings? As extremely unfortunate as it is when they happen (Hashem yerachaim), from the airlines' perspective, they are nothing new. Stabbings happen all over the world, every day, and the airlines don't feel like setting a precedent to simply allow free changes/cancellations anytime there's a spike in local crime. The fact that they are acts of terror (so-called "politically motivated") as opposed to plain-old-fashioned crime doesn't really impress the average airline bean-counter, unfortunately.