where does this freedom of movement come from, what prevents the monarch from locking you up if you want to leave? or why would another let you in?

i dunno, but imo if you're going to be idealistic about people's behavior, anarchy and self-sovereignty seems a better tree to bark up to, at least it's based on your own responsibility 

 The monarchs that opress their people will fall in short order due to their inability to compete in a free market.