This is becoming a common reaction in the UK, not just at borders and not just with customs and immigration officers. I encountered a similar attitude once when I was accosted by a security guard in Tesco who demanded that I put shoes on. When I politely asked him why he immediately called his supervisor on the radio and told her that I was threatening him.
Most likely though it's a health and safety policy - sharp things on the floor, etc.
Try it!
We are, after all, a species capable of some pretty astonishing levels of savagery, when we really put our minds to it. This is true of any ape, of course, but we're clever. The primary reason for social rules and norms to exist is to restrain us, by some means short of battery and homicide, from exercising the inventive nastiness that is our heritage. Of course one may break some such rules, like that about shoes in public places, and harm no one - but an important secondary function of compliance is in signaling probable harmlessness: "I follow the same rules you do," it says, "and therefore will not hurt you." Of course this may be false or even deceptive, as when serial killers use it to improve their ability to obtain victims. But the signal is itself meaningful, and the choice not to present it likewise.
Break what rules you will, to disregard the importance of such nonverbal signals is frankly foolish - potentially dangerous, too, in that you may through ignorance or carelessness betray yourself into another situation that poses you a real risk of harm. I don't know whether you still omit footwear at Tesco, but if so, perhaps at least now you'll better understand why people look at you funny when you do.
It actually sounds like a cognitive fault where an old instinct (that used to be useful) is counterproductive in the modern world.
Coddling that behavior leads us (pretty much invariably) to tragedy.
But again, a "signal" with a high false-positive rate (eg, most people not wearing shoes pose no threat) and false-negative rate (eg, most criminals, especially the dangerous ones wear shoes) is not a good or meaningful signal. You can't make reliable predictions of danger from it, while attempting to distracts from more useful signals and gives us false confidence.
So not only does it not reveal danger, it gives us false confidence we can reliably detect danger, making us more vulnerable to real danger (eg, serial killers).
The instinct was likely useful as primitive humans (eg, preferring our species or pack to other species or packs), but likely is a detriment in modern society.
It's also how humans work, and it is worth bearing in mind in discussions like these that our entire recorded history has yet to recount any evolutionary meaningful period of time. Confusing how humans in modern society ideally should work, with how humans actually do work, leads nowhere productive that I've ever seen.
It's not, it's a detrimental evolutionary holdover we should (attempt to) train ourselves to ignore.
Much like the behaviors that lead to obesity.
Perhaps one day, too, there will exist a human or posthuman being of sufficient perceptive, cognitive, and mnemonic capacity to understand how our species and our societies work in sufficient detail and completeness to usefully contemplate its directed modification. That day also has yet to come, and judging by the results of those attempts at social engineering our histories document, I'd have to say that to suggest any such course of action is, in the most charitable interpretation, an expression of optimism very far beyond the possible.
Until such a day arrives - and I suspect we'll know it when it does - I might make so bold as to opine that a much more productive avenue for our efforts would instead be to perceive and understand how our species and our societies actually do work, with an eye toward optimizing our own behavior toward whatever goal we find best suits us, while not making ourselves so intolerable to our fellows that opprobrium becomes condign.
Granted, this requires not only humility but the total abandonment of faith in human perfectibility and all other such nonsense beliefs - neither of which changes is terribly palatable at first blush. I have, though, in my experiments thus far toward modeling my own behavior after this apparently heterodox ideal, observed the results to be quite strongly beneficial in my own case, and to the limit of my perceptual ability, in the cases of those who regularly interact with me as well. Perhaps others might observe a similar result.
At its core, it is precisely the Christian doctrine of the new life in the world to come, only with the serial numbers filed off, and God and the eschaton elided from the picture. As such, it makes no bloody sense whatsoever, but still has a strong appeal for Unitarians and others uncomfortable with religion and thus unavailing of the Christian utopianism of which "human perfectibility" is a corruption.
...especially since being barefoot makes you less likely to start trouble for practical reasons anyway.
It sounds like a way to sidestep the security theatre somewhat.
It might have had very much to do with it though, as in, the person refused body scan, and the TSA agent, who now had a chance to focus on them more, found them suspicious.
(In that, if they just went on an complied with the full body scan, the agent might not have thought about them twice).
There is no readily available linkage between my presence at the pat down and my identity. I have never had to identify myself for scanning or for pat down.
It would take some effort to link me, either via facial recognition or surveillance tracking back or forwards to a sign-in location where my identity is known.
After opting out 100% of the time for years, I've had my bags inspected by TSA (the little note inside) only twice (out of probably 10 domestic trips and 40 international trips). Both times were international and both times were where I was carrying random wires, Arduino experiment kits, etc. (things that probably look suspicious on an Xray).
I doubt either had to do with opting out.
It's common to get a mark on your bags (marking you for further checks) as they pass through the belt, which can be put there depending on whether or not you seemed suspicious to the guards. No need to actively identify yourself for that.
This is false. I apologize in advance for not being able to substantiate this beyond my own anecdote, but I was once flying cross country, at the part where they scan your ID before standing in line to have your genitals irradiated. A part of the agent's screen that isn't normally visible was facing me for whatever reason, and when my ID was scanned, "opt out requested" or similar appeared in green text.
I had previously also been 100% opt out. Presumably they were referring to times I had opted out in the past.
I won't speculate about how they link this data, it's not terribly hard, but FWIW, they know, and keep track.
I've never gotten a TSA ticket, despite also flying with lots of questionable-looking microcontrollers. I've even gone so far as to leave a note in the bag explaining what each piece was.