This is a fine question. And the point is well-taken, regarding the seriousness of the offense of smuggling diamonds. Yet the title is confusing to me. In what way is it an award to keep the smuggled diamonds? It seems like rationalization for an act of questionable behavior to consider this an award.
The analogy with illegal transport of narcotics is helpful in that it clarifies the significance of Shimon's dilemma (to keep the diamonds or return them). At first glance it seems a limited purpose analogy. If it were an illegal narcotic and not diamonds, then Shimon would in fact be committing a crime as well by keeping the contraband. Yet the analogy does hold water! Why? Because the diamonds may or may not be legally owned by Reuven. So if Shimon chooses to keep the diamonds, he incurs some (or possibly all) of the risk as if the contraband were narcotics instead of diamonds.
Shimon might be putting his friend Reuven at risk by keeping the diamonds. Would the receiving party of the diamonds take action against Reuven? And since Reuven knows that Shimon has possession of the diamonds, because of the phone call, will this receiving party then demand that Shimon deliver what the receiving party may now consider that party's property?
The matter of whether or not the diamonds were discovered before or after the flight is very relevant. See the comment by @Curiouser in the linked question Diamonds in the coffee . This is the reason: If Shimon made the discovery after the flight, and decides to keep the diamonds, he is choosing to deliberately disobey the laws of the State of Israel in not one, but two ways. First, he is breaking laws about declaration of goods when crossing international borders. Secondly, he is keeping money that belongs to the Israeli Treasury, in the form of taxes (tariffs?) that would be owed if such valuable goods were legally transported.
This is probably not the sort of answer that you wanted. I scanned through Deuteronomy and Leviticus for a little while, for guidance by analogy, but to no avail. I am a poor Jewish widow, and do not have the knowledge of Talmud as others here.
I see this as a matter of Reuven behaving very unjustly to someone he considers a friend. Shimon has clearly been treated unethically, put at risk and exploited. He deserves recompense. Should it be the eight diamonds? Probably not, due to the reasons I described. What should it be, and of what amount? Something of sufficient monetary value that will discourage Reuven from acting like this again in the future.