I agree that limitations on benefits and reward flight availability should certainly be more clearly stated but I do understand the rationale for denying the same day change perk for reward flights.

Only a certain number of seats are released for reward flights, allowing free same day change on certain routes is potentially opening up circumventing this from EIs perspective (e.g. the earlier flight I want has no reward availability so I'll just book the later one and change on the day). One could argue that there is no availability for a same day switch ever on a reward booking as there is never an option to book a reward flight for a same day departure so therefore no availability to move flights but as i said the easiest option is just to make clear that this perk is not available for reward flights.