Getting exposure for your ideas is worth much more than however much money he would make on the book sales.

It's a very intriguing question. But yet we see examples of people with exceptional personal wealth, doing things and saying things that seem ... petty or at least beneath them in the financial context.

I think even billionaires want their causes to succeed, so even if it's a book that accounts for 1% of their wealth, they still want it to do well and will say and do things that help it to succeed.

Also, as AngrySkillzz pointed out, the "ideas" put forth in the book are more important to them (than the money earned) and they can indirectly benefit their business by having these ideas, or opinions take hold.