Ok, I agree that confiscating a bladeless Sgian Don't is a little silly, but a plastic knife can be a significant threat. Look at the knives here: http://www.coldsteel.com/nise.html

Notice that one of them is in fact a sgian dhu. I own one of these and, and as a test with the head of security's knowledge, walked through a checkpoint carrying it without setting off the metal detectors. It is a very serious weapon, and while it doesn't have the durability of a steel blade would do the job just as well. Airport security guards in any country have neither the training nor the time to make judgement calls on the seriousness of the threat in anything remotely resembling a weapon, and should err on the side of caution. Pack them in your checked luggage.
Geoff Withnell