And why does Heritage Auctions put them in their own category?
Before they became a US territory in 1900, the islands of Hawaii had been unified into a kingdom that existed for nearly a century. The Kingdom of Hawaii issued their own coinage, cents in 1847 and a series of silver coins in 1883. The cents were struck by a private firm in Massachusetts and the silver dimes, quarters, halves and dollars were designed by Charles Barber and were produced at the San Francisco Mint. These issues are what I consider to be the coins of Hawaii.
Even though Hawaii is now a US state, I think of the coins of Hawaii as "world" coins and would expect to see them in world coin auctions just as I expect to see the coins of Puerto Rico and the coins of the Philippines (although I admit the argument for including the US produced coins of the Philippines in US coin auctions is compelling). However, if you browse a Heritage world coin auction you will typically see the top categories as Ancient coins, World coins and Coins of Hawaii. I don't have an answer for why they have their own category but I imagine it has to do with bidding action.
I have gotten used to seeing the coins of Hawaii in their own Heritage category but lately I have observed a trend that I personally do not care for. Within the Coins of Hawaii category, Heritage has started to include bullion "medals", with Hawaiian themes issued by a company calling themselves the Royal Hawaiian Mint. Some of these may have a connection to a State of Hawaii government office but I believe the majority are strictly private issues. Now there's nothing wrong with collecting exonumia; I just find their placement in the same category to be potentially confusing.
Now that you know a bit of the history of the official coins of the Kingdom of Hawaii, please understand the difference when you come across a Hawaiian themed medal, regardless how "royal" it seems.
Here's my example of the silver dollar (akahi dala).