HKG_Flyer1,
International passengers arriving the USA are required to clear customs, and immigration at their first port-of-call in the USA. Since passing through customs involves access to checked luggage, security is then repeated if the passenger is flying onward within the USA.
The only exceptions to this rule are some Canadian and Caribbean airports., where passengers "pre-clear" US Customs and Immigration. These flights can arrive as domestic flights and passengers may directly enter sterile concourses.
Examples:
Sao Paulo-Atlanta-San Diego:
- Security @ Sao Paulo
- Flight to Atlanta
- US Immigration
- US Customs
- Security @ Atlanta
- Proceed upstairs for connecting flight to San Diego
Montreal-Atlanta-San Diego
- US Immigration @ Montreal
- US Customs @ Montreal
- Security @ Montreal
- Flight to Atlanta
- Flight to San Diego
You have an important, valid point about security and its weaknesses, but one cannot merely clear security in a weak or "suspect" foreign airport and enter directly into the sterile area of a US airport.