When they took apart the phone, they chose NOT to take off the casings that would show the juicy information like the processor being used. Why? Because they needed to be able to rebuild the phone and return it to Apple. They knew from Day 1 it was Apple's phone and chose to profit off the classified information while still being able to give back Apple a working phone hoping that would absolve them from any legal problems. Yeah, not so much.
Also, this depends on your definition of "stolen". If I go to a bar, find a phone there, not try to locate the owner or give it to the staff there should the owner return, and then try to sell it...yeah, I would call that stealing. Do you all agree you would not be happy if you walked away from your phone for 10 minutes and someone called "finders keepers"?