At what point does the mere presence of two people mean one is excerting his will...even just by agreeing? At what point is denying you are exerting your will in a physical interaction...just simply *denial*?
I have been thinking about this a lot. I teach children, and I let them throw me all the time. Of course I could stop their technique, but I really want them to learn, so I go with it. If I started to believe that they were really throwing me, I would be in trouble. I think they know that I am letting them do it, but it gives them the right feeling. We can gradually increase the resistance. That's just one way of training, but it seems to be pretty popular and work well for most people.
I think there is a danger of confusing training exercises with real-life applications. We generally don't do real-life applications in the dojo because they are dangerous, so we do a lot of exercises to develop the skills that we would use in a real-life situation. If we deny, however, that these are training exercises (where we are exerting our will to simulate something or create certain training conditions), I think that can create a bad situation.
Incidentally, I would call standing in one place and letting someone try to throw you or move you an exercise as well. In real life, the mountain that cannot be moved makes a pretty great target for a knife attack.