Hmm, I see the logic there, but I'm not sure it makes sense to me. Negating power claim because you have no place to put it... I suppose it's kind of like negating military claim if all of your characters cannot be killed.
Remember, the prize for winning a power challenge is a two-power swing: he loses 1, you gain 1. You either get all of the claim effect, or none of it. You don't get "half" of the claim effect by having him lose 1 without you gaining 1. So the logic really isn't all that weird.
The "you can't move from without simultaneously moving to" logic has been applied in other contexts as well - specifically attachments. If you have a card effect that says "take control of an attachment and move it to another eligible character," the attachment stays put if there is no other eligible character. That is because there is no time when moving the attachment that it is not attached to either the "origin card" or the "destination card." If it were, it would always be discarded as an unattached (and therefore illegal) attachment in transit.