How was it stolen? It was lost. The finder tried to return it and when he got turned down with no success, he turned around and sold it. The person who did the wrong doing was the finder and not Chen.
You're right that the finder did the most wrong thing here, by selling the lost phone (at which point, IMO but IANAL, the phone was stolen because the finder no longer had any intention of making any further effort to return the lost phone to the rightful owner). However, Gizmodo/Gawker/Chen are not blameless as buying stolen goods is itself a crime.