RFID is of two (major) distinctions, active and passive. For most bar codes, it is passive. The laser hits it, it tells the reader the information encoded (there's details here that don't perfectly fit passive RFID always but they aren't important). This usually is to look up the object and price and after you pay the store uses that information for inventory management. The object does not know if it is sold by itself. For expensive things, the tag is active, and by this it means you don't have to hit it with a laser, it calls out when nearby a scanner eg the door. You could have passive ones read at the door and they are trying to figure that out but it is complicated to energize and read all the stuff in your cart accurately, not to mention very expensive today. Anyway, the active tag (or something behaving like active RFID) is removed or de-energized at checkout so it can't call out to/be detected by the machine and set off the alarm. A red bull tag won't do this, nor will 99% of most tags in most stores. Most tags are meant as a way to quickly identify and look up an item.