"I think an interesting question is why it matters so much to Sam Harris (and to his many acolytes, including Edward John) to prove that free will does not exist"

It's just human nature. When a person strongly believes something, we try to convince others of it too. This is not unique to me or Sam Harris.

"Sam says "The illusion of free will is itself an illusion" -- when we are always told at the end of the chapter that the illusion is inevitable. We will both continue to believe we have it and society will treat us as if we have it."

It is inevitable then that some people will try to convince others that there is no free will. And some of those will have their minds changed by that and some won't.

"No, I'm sorry, in any meaningful sense, they are all involuntary unless some version of free will exists. Harris's distinction is the sane as John's, where we choose not to treat the serial killer's behavior as truly voluntary because he had a brain tumor. But as John argues, we are all that serial killer, predetermined to do exactly what we did and do. The killer did not choose to kill in any meaningful sense and we don't choose any of our actions either. When Harris and John try to hold onto "choice" they are defining "choosing" as "what you did.""

Yes, I see what you mean. But we still have an understanding of the difference between voluntary and involuntary. For example, if I fall down the stairs, it's invountary, but if I jump down the stairs it's voluntary. We can understand the difference between those two actions and their meanings and yet still understand that we don't ultimately have free will.

And it does make a difference to how we view that person. If someone deliberately jumps down the stairs, this tells us something about their mental state. They might be suicidal, or trying to get attention/sympathy. But if someone just falls down the stairs, it might just mean they are clumsy.