Not a huge leap at all. The civil statute says that you can't keep lost property - you MUST turn over anything valued at over $100 to the police within a reasonable period of time. The only way you avoid this obligation is if you choose NOT to take the lost property into your possession. The "finder" elected to do so. He could have given it immediately to the bar - and then they would have the obligation. He definitely didn't turn it over to the police - he kept it and showed the absolute intent that he wanted it to be his by selling it. If he sold it for $399, he might be facing a misdemeanor, but he established the felony value of $400 or more by selling it for $5,000. Keeping it and selling it violate one penal code listed above; acquiring it as Gizmodo did appears to violate another.
Maximum liability is a year in jail under the applicable California statutes - although it might have been less if he didn't value it at $5,000. If he kept it and thought it a regular sub-$399 phone, he would have been liable for a misdemeanor charge.
Sorry, but it's not a crime to fail to comply with a civil statute. That's why it's called a "civil" statute. Complying with the penal code requires making a reasonable effort; what is reasonable is up to the jury. It doesn't matter what a civil statute says.
This is a *basic* fundamental principle of criminal law and of (US) constitutional law.