Take the example of a character who is unconscious. He is incapable of making a choice.
Against Cure Light Wounds, a [harmless] spell which allows a save, he attempts no save; there is a saving throw against the spell, but because it is [harmless], it requires the target to elect to make that save. In the absence of such a choice, the save is foregone automatically... and because he's unconscious, he cannot choose.
Against Inflict Light Wounds, a non-[harmless] spell which allows a save, he automatically attempts the save; there is a saving throw against the spell, and since it is not [harmless], giving up that save requires the target to elect to do so. In the absence of such a choice, the save is attempted automatically... and because he's unconscious, he cannot choose.
So, similarly, when the cleric says to you "I'm going to cast Cure Light Wounds on you, so don't bother attempting a save", you don't need to make the conscious choice "I will forego this save"; rather, you make no choice at all. If it is, indeed, a [harmless] spell, it will take effect automatically, since you have not elected to attempt a save. If it is, however, actually a non-[harmless] spell that allows a save, like Slay Living, you will attempt a save automatically, since you have not elected to forego your save.
-Hyp.