You're assuming that everyone who wanted a nav system would either buy one new or buy the factory add-on. That's not what would happen. People would stop buying the factory system new and would instead buy the hacked one from someone for half the price and Ford gets nothing.
This way the people who want factory Nav will buy it from the factory. The ones that didn't get it from the factory but are willing to pay factory prices for it to be added later are few and far between.