They have watched Hollywood too much

These "terrorists" watched Hollywood movies too much. Every car in the movies hit by a bullet explodes violently (unless the hero is driving, then it can be riddled with AK-47 or even .50 machine gun bullets with only damage to the windscreen). Had they watched Mythbusters, or ANY program on Hollywood special effects, and been much the wiser. Unfortunatey, many of our leaders appear to suffer from the same belief in Hollywood-movie style physics and chemistry (and hence impossible plots).

Having said that, it IS fortunate that these particular "terrorists" who hate others sufficiently to want to blow them up or burn them alive were this incompetent. Had they been as competent as, say the Oklahoma City bomber, who made a decidedly low-tech, but deadly car bomb, without blowing himself up, London and Glasgow would have lost some buildings, and, more importantly, many lives.

So yes, vigilance is useful, but paranoia (such as the measures concerning liquids on airplanes) is another matter entirely.