If you read the letter of the law, any magazine with a capacity over 10 rounds capable of being inserted into a pistol is illegal. Some here argue that, effectively, that means any magazine, period, because someone, somewhere is capable of making a pistol that can accept that magazine.
Others believe that if there is no commercially manufactured pistol variant, or if the pistol variant is banned in Hawaii, then it is ok to possess larger capacity magazines for a rifle. I've heard more than one officer in the firearms division of HPD say that Ar and AK mags of any capacity are fine.
But what an officer says really isn't relevant, nor is the opinion of people on the forum. It's the stance that the state prosecutor's office takes. And, to date, there has been no official opinion issued.
(c) The manufacture, possession, sale, barter, trade, gift, transfer, or acquisition of detachable ammunition magazines with a capacity in excess of ten rounds which are designed for or capable of use with a pistol is prohibited. This subsection shall not apply to magazines originally designed to accept more than ten rounds of ammunition which have been modified to accept no more than ten rounds and which are not capable of being readily restored to a capacity of more than ten rounds.
So, are the gun stores selling illegal magazines? If you take the wording of HRS 134 literally, then yes. It is clearly, explicitly illegal. The fact that HPD says "it's ok" doesn't make it so. They are, as a matter of purely internal policy, choosing not enforcing that statute.