This is another issue that no one challenged yet. The HRS doesn't state only spouse is allowed to be joint. But everyone is told only spouses can be on by HPD.
I think part of the issue there is the assumption that firearm registration indicates ownership.
In reality, the name/s on a registration isn't a guarantee of who owns the firearms.
When you pay a dealer for a firearm, you are the legal owner of that property. However, it can take months before you can legally take possession of a firearm. Until then, you are still the owner WITHOUT the existence of a state-filed registration.
When you register someone else on your firearm, there is an assumption that you both own the firearm equally. But, if I put my buddy's name on the form just so he can borrow it now and then, he has no legal claim to my property unless I give him a separate paper saying he and I are joint owners.
Limiting the joint registrations to spouses is one way of making sure the people on the registration form can be assumed to be joint owners. States have joint property laws for spouses that avoid problems trying to establish ownership.
That's my guess. Logic rarely applies to HPD's policies.