Your right. He did remove them without permission. Does it warrant 20 years in jail considering he returned them (documents not really even important in the case) upon demand?
People just love seeing the conviction of those they envy.
He knew he was not supposed to remove the documents, and he did it anyway. I don't know why you're inclined to condone direct violations of court orders, but that is what happened.
The conviction has nothing to do with envy. It's about fraud, by Black against his companies' shareholders.
To convict for mail fraud, it is necessary for the jury to conclude that there was a fraud and that that fraud involved the mail. Use of the mail in furtherance of the fraud is was makes it federal rather than state law. In any event, the fraud is proven as part of the offense.