You can summarize this opinion fairly easily:
Give the guy back his weed.
It's not illegal under state law, so the state can't keep (or destroy) it if he's not charged with a crime. The fact that it's still illegal under federal law doesn't matter.
So that's 21 grams back to Mr. Smith.
P.S. - Fun fact: You could always tell the law students with misspent youths because they were able to convert grams into ounces in their head. I'm thinking of a former roommate in particular. Whereas most of us have to look it up to discover that 21 grams is around three quarters of an ounce. (Though knowing just how much an ounce of marijuana entails may itself demonstrate some prior knowledge of the subject matter.)
