Josh, one could also ask, why offer Action Items in the first place?
There are instances where you would want an activity to be associated with a specific work item, but not directly on the project itself. For example, let's say I have a meeting task on a project that has follow up items. I would want to connect those follow up items to the meeting task, but not include them in the official work plan. Another example, I have work items that have a number of smaller steps involved that we may want to document and have the ability to mark it off as done (like a checklist), but I wouldn't want to clutter up the work plan with 20 minor tasks that make up the actual task.
That said, I agree with the OP's enhancement request because Action Items feel a bit useless with their current functionality.