To look at it another way, imagine that your question is "If I paid for a movie on videocassette, why do I also have to pay for the DVD or the novel?" Licenses to use intellectual property may specify the format, manner, platform, and venue (among other things) for use of that property. You have certain Fair Use rights as well, but this is not one of them.
IANALNDIPOOTV. YMMV.
--Chris