higher concentration of people such as shopping mall and sporting events are because airplanes are high value target and can be used as weapon of mass destruction. Once you have enough security that bombs and weapons can not get on board of plane, what's the point of more security? If it is to protect the people than all areas with high concentration of people should be looked at and it is simply impossible to do so w/out becoming a police state. The best defense against attack like this is better intelligence at infiltrating those terror cells. But even that might be more difficult as just 1 or 2 people can cause so much damage.