The US copyright's determination was based on the simple analogy of someone hiring someone else to create a work for them. The person hiring, even if they offer suggestions and veto results, is not contributing enough to the expression and therefore has no right to claim copyright themselves.
If you stand behind a painter and tell them what to do, you don't have any claim to copyright as the painter is still the author of the expression, not you. You must have a hand in the physical expression by painting yourself.
