Shop lifting scumbags

I've decided that I'll never understand what motivates some people. So far, I've had 2 cases (that I know of) of shoplifting in the new hobby shop. The first one I only found out about by accident, when I went looking for an item I knew was there a week ago, but had mysteriously vanished without having been sold. Sneaky bastards got away clean on that one!
The second one was committed by one of two blokes who I'd bent over backwards to help. To summarise events, they came in and purchased 2 indoor electric helicopters, but managed to lose one outside in the wind (after being told not to fly them outside in the wind......) at an altitude of about 60 meters and some 6 blocks away from themselves. I gave them a replacement the next day at cost price. When I ducked out the back to see if I had some spare blades that I was going to give them, free of charge, one of them decided to rip open a box and steal the spare blades out of a new one! Another customer who was in the shop at the time told me what had happened, but only after the thieving bastards had left (probably a good thing anyway, or I'd be speaking to the police for entirely different reasons, ie assault charges) :-( Now, I can accept that some random someone might come into the shop with the intention of stealing, but after the help I'd given them, it was the last thing I expected them to do!