OK so we know this is potentially a real threat and quite doable by motivated individuals...So what is the Government doing about it? Anything?
If they already have the weapons and are in place with orders to use them, there isn't anything the government *can* do, other than annex, raze, and pave a 40 mile radius around every airport.
Or, we can do what we're doing now, and go hunt down the people who organize, train, and order these attacks. Suppressing the regimes that might sell such things works well too as these things have a limited lifespan.