It's a very sad world we live in. Throughout the years I've been flying I've seen so many things and I still get shocked by them.
Here's a list of things that have "walked" off the aircraft and no it wasn't me - crew bags, crew uniform pieces ( jackets etc), money from duty free, food from the galley, alcohol from the bar, life jackets, seatbelts ( don't ask me why ), toiletries from First and Business class toilets, glassware and cutlery from the trays, safety cards, magazines, blankets, pillows and the list just goes on and on. I swear if they could take us too they would.
Only last week I turned away from the cart to answer a pax question and when I turned back a man had his hand inside the wine draw helping himself. The airline I work for the pax have to buy food and drink so no it wasn't complimentary.
The problem is temptation and when opportunity presents itself these people can't resist. I don't think it's any different in other industries, look at hospitality, sales, hotels or hospitals it happens there too.