I see what you are saying but work for hire cannot be implied, it has to be explicitly written in the agreement. There is no confusion when you are doing a job "Work for Hire."
AND the work must fit into the nine categories specified by the law, or it still won't pass muster as "work for hire." If it doesn't fit within those categories, the contract must explicitly state that the copyright is being transferred to the commissioner/employer.
This doesn't mean the photographer is always free and clear, though. Copyright is a federal matter. Depending on the agreement (written and otherwise) there can still be some state court issues, such as having made a "bad faith" agreement or misrepresentation or issues regarding payment, et cetera. Conceivably, the photographer could own the copyright but be enjoined against using it for other reasons.