This is actually a US Federal Requirement, which long pre-dates September 11th. It applies to US-based carriers only. The foreign carriers have much more leeway.

All flights to the US have a variety of specific rules, based on the risk category of the foreign airport. Certain airports overseas (particularly in Europe) are designated as Category "X" or "Extraordinary Risk."

In some cases, there is mandatory secondary screening: bag inspection and frisking for all passengers at the gate (this was the case at Brussels for quite some time.) In other cases (Frankfurt) there is sometimes a specific second checkpoint on the concourse for all US-bound passenger only. In most cases, (Paris, Sao Paulo, for example) there is random secondary screening at the gate for US-bound flights (frisking, bag inspection, explosive swabbing.) In all cases, there may be specific screening for selectees.

All European and South American airports require passengers to undergo two interviews: prior to check-in and at the gate.

Many foreign airports have 100% liquid screening (Tel Aviv, Hong Kong), but do not typically have frisking or explosive detection at the gate or on the Jetway.

If you're flying on a non-US airline, you may still be asked to submit to an interview (Air France does this for flights to Israel and the US), or some of the same random screenings may apply. In other instances (Cathay Pacific), there is liquid screening only.