Even though 9/11 was carried out by foreign extremists - everyday Americans are treated like potential terror suspects for trying to board an airplane, even though American citizens do not have a history of hijacking or committing acts of terrorism on planes.
The TSA is a result of post 9/11 hysteria in which people have allowed themselves to surrender their rights and their human dignity to Federal Agents in airports in the name of safety. The TSA quickly positioned itself as one of the most authoritarian and power abusing agencies within the entire United States security establishment.
There is no way to be 100% safe. You can get stabbed walking down the street. You could be in a store that gets robbed. Someone could bomb a bus or a train or a hotel or a mall. Someone could at any time commit any number of heinous acts and kill people. This is an unfortunate reality. However, the answer to those very real potentials is not to start searching people randomly on the street, or groping people who want to come into a store, or making people go through a full body scanner to shop in the mall.
Security must be efficient and reasonable - not abusive and humiliating.