I would have to go back and look through the articles, but did they use the word "registered" or did they say they traced it back to him?
When you buy one, you simply open the package, charge the phone, and enter the code or number to load the minutes. That's it. You start using it. If you run out of minutes, you go back to a store and buy a card with more minutes, and just load the code/#. No names, no registration. IMO, LE accessed the Runion's call activity log, and just started calling the numbers, and he answered it. IDK. Perhaps there was more to it. But, it could have been that simple. Also, all cell phones (whether disposable/prepaid or contract) have and internal ID # (ESN, ESID, etc) which is specific to the company, so they could've traced the SN #, then that company would probably be able to find out which store that phone was sold at....then security cam footage could be used to see who was in a store on a specific day.
Also, with these disposable/prepaid, you do have the option to go online an create an account to facilitate adding new minutes, but I doubt if it's being used as a burner/drop phone, the perp would register online. Jmo.