I don't necessarily think that this monetization strategy is the best idea but I'm not going to begrudge the guy for trying to make money from his business regardless of whether he already has a lot of money.
This is akin to asking why Sergey Brin charges for AdWords when he's already a billionaire or why the richest person in the world still makes money off of Windows. Your logic is moronic.
http://www.quicksprout.com/2009/04/08/the-andrew-warner-stor...
So he's charging because he can, not because he must.