You sound as though they have some choice as to which box to take

Do I? That wasn't my intention. They don't have a choice in which box to take, any more than they have a choice in whether or not they find my argument compelling. If they find my argument compelling then (if they are rational) they will take 1 box and win $1M. If they don't, then (maybe) they won't. There's no real "choice" involved (though there is the very compelling illusion of choice).

This is actually a perfect illustration of the limits of free will even in our own awareness: you can't decide whether to find a particular argument compelling or not, it's something that just happens to you.