"And 'focusing' attention on the Twin Towers (and every other tall building) would have been any easier?"

Far from it. Frankingly, I'm convinced it's not completely possible to protect against every threat, not without doing things that make the Patriot Act look liberal. It's not that drug smuggling, illegal immigration, and the like tell us our security is lax, they tell us it can't be done. You do have to focus on the likely threats and put more emphasis on human intelligence at the likely source of any threat. Better security at our borders is the second line of defense at best and a relatively weak one at that. The bad news is, much as we've learned with people determined to assasinate presidents, if someone wants to attack us badly enough, they'll find a way.