Actually I wondered about all this when I got a SIM (Movistar) a few months back. My DNI was accepted and we were off and running, but I remember no option that said you could use a passport number or any other form of identity verification than a DNI to implement the phone service.
This suggested tourists in-country on a visa are left either paying steep international roving fees or depending on the likes of Skype. Maybe another temp carrier not Movistar is more forgiving? A government requirement I'm guessing though.