As far as I've seen, this only happens when you help someone who is acting on the same turn as you. If you help someone who goes after you, then they still have their full actions. Thus I interpret it not as standing up costs an action, but that you've already spent your "action" making a death saving throw. Which kind of makes sense?
If your heart stopped, then someone defibrillated you, would you immediately be ready to fight (or do some other action-equivalent thing). Or would it take you at least a few seconds (a turn) to get your bearings?
