I've thought about it, but It gets complicated really fast form a programming standpoint. Who should pay for the update, who just bought the book and shouldn't? How much of an update constitutes a fee? All that kind of stuff. And, the big one - how many people purchase the book in part because of the free updates? Charging for updates may ultimately cost me more than not.
With luck, Nikon will get to a point where their mirrorless system is more mature and things will slow down a bit. I think they need to, not just because of me
but because it's a pain for users to constantly be learning new controls, deciding how to implement them, and readjusting their field workflow.