Rapaire, you reckon that's bad, consider duty-free. I can use my card to buy a dozen bottles of spirits which will burn just fine. That lot going up is not going to do a whole lot of good to the plane or its occupants. And then there's the simple fact of the bottles being glass and ideally suited as stabbing weapons once broken.
I had the pocket-knife experience on my last flight from Heathrow, due to a truly outstanding bit of incompetence. Heathrow had neglected to hire enough staff, so there were queues the length of the building. I arrived 2.5hrs before takeoff - I got checked in 15 minutes before. This was now too late to get my luggage in the hold with the rest of the luggage, so I had to carry it to the gate and get the attendants to stash it for me. Trouble is, I'd got my Leatherman in my hold luggage (as you do, since you can't carry it with you), and the security cretins wouldn't let me take that into the terminal; neither would they (a) hold it until I got back, (b) issue me with a receipt to say they'd confiscated it so I could claim on insurance, (c) post it back home, or (d) carry it themselves to the gate and give it to the attendents themselves. In other words, they'd basically said they were going to steal it for themselves.
Mind you, US immigration wasn't much better. The conversation there went:-
- What are you doing in the US?
- I've come to do some testing on the software I've written for Ford.
- How long are you staying?
- A week, maybe two.
- WHY DON'T YOU KNOW?!?!?!
So in answer to why they do it, my best guess is that they've been issued orders and very few of them are smart enough to use intelligence in carrying out those orders.