Absolutely yes, Motorola could prosecute. Sorry to burst people's bubbles around here but you can be charged with
possession of stolen property, most likely a misdemeanor but could be brought up to a felony under the correct circumstances. Of course they would have to prove that you knew that the property was stolen, most likely, to succeed in prosecuting you.
In this case, would a reasonable person suspect that the phone could have been stolen since it hadn't been released and it was clearly labelled on the phone "not for sale?" A prosecutor could argue this.
Yes Motorola would have to spend more money than this would actually be worth to them in bad publicity and hassle. If they really wanted to get nasty about it, they could charge you with possession of stolen property, and the courts would determine whether or not they had a case, whether it's a misdemeanor or a felony, what if any time you would serve, and what your restitution fee would be, etc.
I work with young people who knowingly receive stolen property often and I'm compelled to educate them about this law. My young people would be especially vulnerable if charged. You may not be as vulnerable (depending on whether or not you have a criminal record, whether or not you can afford a good lawyer, age, standing in the community, and reputation, etc.) as the young people with whom I work.
Shawheim, you made the right decision to send it back.
That being said, Motorola could be more gracious about this. It's not like he was selling the software or hardware to Motorola's competitors. I guess they're assuming that this could have happened and are acting accordingly.