The honest truth to why they do it is because they don't want to be in a situation where they're shipping keys around between locations (or even pretending to inventory/store them) because people keep forgetting to bring back both or lose one and the other one is in an office in another state. You're far less likely to lose both keys than just one because of how obnoxious it is. And having 2 keys is actually a reasonable increase in re-sale/auction value because of how expensive/difficult it is to re-key cars these days (unless you're a criminal then it's really easy and cheap apparently).