I suppose if he were to sell the PDF, it would automatically turn his "encourag[ing] redistribut[ing] the PDF, including via BitTorrent" into piracy. If you see piracy as a necessary evil, you might grumble at that; if, like Minecraft's creator, you see piracy as a nullity:

http://www.next-gen.biz/news/gdc-2011-piracy-is-not-theft-sa...

and say "there's no such thing as a lost sale", then you might not care in the slightest. But the nature of the ecosystem surrounding your book will inevitably change: as someone said elsewhere in this thread, you can't loan an eBook; a solution to that would be sending someone the PDF, but you've just turned them into a pirate.

There's no easy solution. Upselling blogposts might be a better idea; even then, it would be good to have figures.

An even more radical alternative would have been Mark Pilgrim's approach to Dive Into Python. You could give your book a Creative Commons licence and expect people to copy and remix it:

http://diveintomark.org/archives/2009/10/19/the-point

but then Pilgrim admits he's not in it for the money, so in the absence of reliable financials: who knows?