Or maybe he simply doesn't want people paying him for free things (I imagine in some cultures that's offensive, besides the fact his own personal opinion is the important one)
Or maybe he doesn't want to be bribeable for requests (you've seen this: free app maybe even with ads = ehhh poor guy does it in his free time, he can't be everyone's personal hotine; 99 cent or higher app = I fucking paid for this and expect full service)
Or maybe some/all of the above?