In many European airports, it's common to "shunt" passengers with very short connections. A bus or van meets the incoming aircraft, collects the passengers, and drives them to their next flight.

Here is my question: how do the airlines and airports permit these passengers to skip transit security?

I don't have a specific answer; however, I think it's a matter of leaving what is defined as the sterile area. Once you leave the sterile area, you must be re-screened. Therefore, if a passenger gets off an airplane and is driven directly to another airplane or gate without ever leaving the sterile area, then it is not necessary to be re-screened. Of course, there are specific exceptions for those originating airports that do not meet TSA standards for screening. This mostly applies to certain foreign airports.

Bart,
I was actually speaking of airports that require re-screening even if the passenger stays in a sterile area. For example, passengers transiting in London, Brussels, Copenhagen, Oslo, Paris, Tokyo, and many other points must be rescreened.

These passengers do not have access to checked baggage and do not exit the sterile area.

The airports just require that anyone transiting in their airport go through security screening.

In some cases, such as Amsterdam, Singapore, and Vienna, the checkpoints are at the gates themselves, so an "express van" wouldn't make a difference. The passengers would be rescreened at their departure gate.

But what about airports with centralized screening? Are these passengers exempt from this procedure?

-Mat.

Surely the point is that if you remain in a sterile area you shouldnt need to be rescreened. I know that the U.K categorises its arriving flights according to where they have arrived from, "Dirty flights" are subjected to additional security measures as opposed to flights from the US for example which are considered clean

Hey, don't call me "Surely." (ha ha).

That's interesting about Gatwick. I've had to be rescreened as an arriving passenger from the USA. But a few years have passed, so maybe times have changed.