Another possibility is they're straight up lying to make him look bad. I can't see why they wouldn't charge him with something that is so easy to get a harsh sentence on though if there's any real evidence of it. Contraband cases are like drugs, if they have the contraband, they have the conviction. If they don't have the contraband, either because they literally physically don't have it or can't introduce it into evidence, they don't.
So barring it being unprosecutable or something else, such as the feds outright lying (which has been known to happen) it would be a peculiar choice not to prosecute.