My father-in-law keeps $ that does not belong to him!

I was at his house last Sat for a visit when mother-in-law told me that a cashier at SS supermarket had accidentally given my father-in-law more $ than she should have. So I asked her if he returned the $ to the cashier. "Of course not!" came the reply. "Huh? Why?" I asked. "Why should he when he can get to keep the $???" Oh my gosh. I was disgusted. It was not a matter of a few cents but $20-$30! Now the cashier would have to pay out of her own pocket. When I told my mother-in-law that she may get retribution, she said "No, no. Keeping $ given to us is not a crime. After all, it is not as if I robbed someone". I was dumbfounded. Why did my father-in-law have to do this? It is not as if no one gives him any $! My hubby, his twin brother & his 2 sisters give him $ on a regular basis. Only his eldest son & 2nd daughter do not give a single cent. (Well, that's another story.) It seems that one can't get elderly people to change their mindset. It is well, like cast in stone.