The fallacy of that justification is that they are not imposing comparably sized exclusion zones for cars and trucks, which could carry a larger payload and attract less attention.
Yes...yes they do. My firm works in political event logistics among other things. Everything from POTUS to national candidates to local candidates. All of the "buffer" zones are to minimize the impact from an attack from an estimated distance from an estimated threat.
That means when he is on a stage, there is a certian amount of feet that the public is kept from him (unless pre screened through DOJ...yes all of those people permitted within that buffer zone are name checked with SS#'s). When he is arriving, unscreened vehicles are kept a certain distance from the drop point minimizing a car or truck explosion (have personalty sealed off a two city block perimeter) and the TFR is designed to keep the a renegade pilot with a grudge from making an impact that could affect his security.
And ALL of those distances have increased significantly post 9/11.
Even the media is not allowed to report on the motorcade route if they know it... and that has multiple contingency plans.
You can be sure if the outter ring of the TFR is penetrated, the command team on the ground is alerted. If they penetrate the inner ring before getting intercepted, you can bet the extraction team (yes, there is such a thing) is sent into action and the principal is removed.
Every possible scenario has multiple layers of protection layers be it people, vehicles or aircraft...and yeah, it is a pain in the arse...for everyone.
My favorite was during a not very high dollar fundraiser with W I was chatting with a tech from the phone company wiring the the heck outta the building and he says to me "this is stupid, they are gonna make less at this "fundraiser" then they are paying our company alone to do all this work.
"Yup" I said. "It is more about marketing and media attention than money sometimes."
Yes these appearances "cost" a lot of money, but my counter argument is that they pump a LOT of money back into the local economy from contract services provided, hotels, overtime of LEO's, transportation services, ect.