A voluntary transaction cannot be robbery. However, to answer your question: the Kindle version offers numerous advantages over the print one. It won't be irretrievably lost if it gets wet, dumped in a fire, or left somewhere. It can be stored on a simple, easily carried device with thousands of other books. If that device is destroyed or lost you can still access the book via any Internet connection. The font and text size can be altered at will. Notes can be entered without need of pen or highlighter. The paper copy has none of those features, hence it costs less.