TSA has more than a bit of scope creep going on. In theory, all they're supposed to do is a basic administrative search to prevent weapons or explosives from being brought onto a plane. In practice, the perception in the public mind that they are a more general law-enforcement body allows them to easily perform other tasks on behalf of agencies that don't have the same general search powers.
The classic example is searching for large sums of cash. Detecting large quantities of traveling cash and ensuring it's been properly declared is CBP's (Customs and Border Protection) job, not TSA's. But because TSA gets to X-ray and search every bag and body-scan every passenger, they catch that stuff, and then hold the passenger and go get CBP.
The result is that the number of outside-the-mandate things that can end up happening as an "incidental" result of TSA's screening is staggering.
Is searching for cash the classic example or a one time example involving Ron Paul's cash courier, which then led to a rule change instructing TSA to ignore cash?
TSA claim they don't search for eg drugs; they claim to search for expsi es and incidentally find drugs. This is the classic mission creep example.
There are people who hide drugs in jars of peanut butter. This shows up on TSA screens as a jar with different stuff in, which they claim to view as suspicious because it looks like explosive.
Well, I do wonder if there's any benefit to the TSA for finding drugs? Perhaps I should trawl theough the reports to see if the TSA uses drug-finds as part of their results to support their work?
Totally anecdotal, but I was once pulled aside on the jetway by a woman with a Treasury badge and a burly guy who was probably a sheriff though he never said anything. She wanted to know how much cash I was carrying. I was flying from SF to Frankfurt, and I think it was 1994. My net worth then was about $30k, so I laughed and told her I had about $200 and a credit card.
So maybe CBP doesn't check outbound travelers, but Treasury did at least once.
CBP can and does in fact carry out checks for undeclared cash exports in the jetway for international departures. Not every departure, obviously, and I don't think the selection mechanism is public, but it's a thing they're documented as doing.
You do get searched by TSA if you are coming from outside the country. In recent years I've flown in from Mexico, Spain, India and the Dominican Republic. For each flight, the boarding gate was blocked from general access and the only way to get in was to go through a TSA search. They only had metal detectors, but my bags were still searched. Had I been carrying a large sum of cash (or anything else that needs to be declared) they could have contacted CBP at my departure airport.
That is not the TSA doing the checks though. In Beijing, there is gate screening just as you described, but it is done by locals who don't speak English, and they definitely aren't looking for cash.
Based on reports from flyertalk and other places, it appears that there is a requirement that a certain minimum percentage of US-bound international passengers undergo at least a TSA-style boarding-pass-and-ID check.
Typically this involves stationing agents at the departure gate. Anecdotal evidence suggests AMS is a popular place for this.
The classic example is searching for large sums of cash. Detecting large quantities of traveling cash and ensuring it's been properly declared is CBP's (Customs and Border Protection) job, not TSA's. But because TSA gets to X-ray and search every bag and body-scan every passenger, they catch that stuff, and then hold the passenger and go get CBP.
The result is that the number of outside-the-mandate things that can end up happening as an "incidental" result of TSA's screening is staggering.