If a terrorist wanted to kill a lot of people confined into too small space to run away, what better target than these densely packed zig-zag queues? Really, what's the point of protecting a plane carrying hundreds of people by putting those same people into harm's way like that? It already happened in China - Muslim terrorists killed dozens of people waiting in a ticket hall a couple of years ago.
There are a large number of places with densely packed people. The unique thing about airports is their access to airplanes and their capacity to be used as weapons. (I'm taking a broad view of "weapon" here; even old-school hijack-and-divert is still a political "weapon".) If it weren't for that, they wouldn't be particularly interesting and wouldn't need special security. If someone blows up the line, well, they di…
The airport itself is a distinct target. A successful attack at an airport would disrupt travel and commerce for weeks, as we just saw in Belgium.
Nothing immediately comes to mind to mitigate this. We can only push so far upstream. We can move the screening to empty fields a mile away from airport structures and require that passengers walk themselves over lest the shuttle bus be appropriated...