[jasong]

The review acts on a project, not on an action, therefore the behavior you're seeing is a side-effect (some would say defect) of the way single-action containers (i.e. projects with the "this is a list of single actions" checkbox checked) are treated by OmniFocus.

Because the actions are technically part of a single project, when you change the reviewed date, you're not changing it for the action, you're changing it for the project. Thus, the entire single-action "project" is marked as reviewed.

If you think this behavior should be changed, please send feedback to OG through the Send Feedback mechanism in OF.