Since marijuana is still illegal at the federal level, I can see the prohibition against people who use it. But just because your doctor prescribed it doesn't mean you ever followed thru and filled the prescription. Simply receiving a prescription isn't illegal, and shouldn't be used as a de facto ban. The onus should be on HPD to prove that you've actually used an illegal narcotic before confiscating your guns.
Federal law prohibits
selling a gun to someone who uses marijuana (or
buying one if you're a user), and the ATF has interpreted that to mean that holders of medical marijuana cards are prohibited persons on the grounds that holding a MMJ card implies use. However, federal law doesn't ban
using a gun you already own, or buying ammo for that gun, if you use marjuana or hold a card.
Your state goes a lot further, and bans possession or use of firearms by medical MJ card holders. As the letter says, a MJ cardholder in Hawaii is actually required to surrender his firearms to the police there. Because your state requires registration of all firearms, it's easy to implement confiscation. Therefore, anyone who owns guns should avoid getting a medical MJ card, or otherwise creating any paper trail of MJ use. This is especially important in those states that also require registration of firearms.
One thing that I've tried to find out but haven't yet been able to learn is the meaning of the "medical doctor's clearance letter" mentioned in the letter.