Since it's *not* PJW whom you're quoting but I ...
So, you want to only purchase the book and determine on your own how much (if any at all) to give to the foundation... without which there would not be a book for you to purchase to begin with.
You want to pay only enough to cover the cost of the food you eat, and want to decide if you help pay the mortgage and utility bills of the restaurant which serves you that food.
Think about that for a bit.
I don't need to think about it, I already did. Was it really the foundation who wrote the book? I have the impression that it was some people who wrote it and, as with any book I buy, I'm perfectly willing to pay its authors whatever part of the book's price it represents.
The same with the restaurant: I'm completely aware that the prices I pay don't go only to buy the food. What I wouldn't like is that what is normally a 20 eur, meal would cost me 40 eur. because "half goes to some charity the owner is very fond of". I very much prefer to decide which charities, foundations, and what not get my money, how much of that money they get and that they get it directly from me.
It would be different if BPM said: "This book is normally and fairly priced but we want you to know that we, out of our own heart, will give half of our proceeds to the FPF". But that's not what is happening, is it? Because then they wouldn't sell you a PDF at the same price that a softcover.
But, of course, all that is
my personal preference; YMMV As I, myself, you're perfectly free to think whatever you want.