All rental car companies charge taxes and fees on top of their base rates -- not just in Hawaii, but everywhere in the United States. That's how it's been for decades. You'll usually see some that are percentages; some are per day, and some are per rental. At Honolulu Airport, expect a Customer Facility Charge, Concession Recovery Fee, Rent Tax Surcharge, Vehicle Registration Fee / Weight Tax, and State Tax (but nothing that;s called a sales tax).
The rental car companies are required to pay, so they collect from the their customers. It's no different than when you pay a sales tax at a department store, except that you pay much for on top of a car rental (as a percentage of the base price) than on a shirt at a department store.
I'm old enough to remember when, in most places, there was just the base rate plus whatever the local sales tax was. Then airports, states, cities, and other taxing agencies figured out that they could collect more money from renters, instead of raising taxes on local residents (local voters). Now, in some places, the taxes and "fees" (essentially more taxes, but sometimes a way for a rental car company to break out a specific business cost) are almost as much as the base rental.
The good news it that rental car websites also quote the total price, including taxes and fees. So just look at the bottom line.
