Well maybe they thought that since he creates bitcoins which helps many people financially but we don't really know if he's kind or what, the only thing we know about him is just a little, No once can even show if satoshi is girl or a boy, And also I agree with you there's no problem if he move that since he have the right to do on his creation and its little percentage of holdings, But if he dump 1000BTC in one day well we can say there's something bad going on.
If he dumps the entire million in an hour, it'd be my time to shine and hoard all coins possible. As soon as you're hodling, it'd be just a little upside down spike you see on your account balance. If you don't sell in panic, you may not even feel it. In fact, you don't lose anything as long as you don't sell.
I'm really wondering whether those hodling are hodling for real if they fear such an event. If that million ever proves to be Satoshi's, then this is a risk that's going to be out there forever. If Satoshi doesn't plan to sell his coins, maybe his grandchildren will.