Isn't it pretty late to re-sell the ticket if you don't show up? It helps them with overbooking, standby customers, fuel costs and increased comfort to other passengers though.
In most cases, they technically already resold your ticket due to overbooking; it makes their lives easier since they don't have to boot someone from the flight, you voluntarily booted yourself. In fact, not only did you boot yourself, you forefeited however much the segment cost you, and any future segments on the ticket. That's why it's so absolutely preposterous that United is threatening to do this. They likely are making money off each person who skips segments, they just don't like it happening because it messes up their ticketing and flight operations models if this starts happening widespread.