It never dawned on me to check the fine print of the rental contract to see if I was breaking the rules. Who does that?
A big part of the problem is that the counter staff at the majors won't go over every detail with you, as they process line-ups of tired visitors arriving on full flights. They will respond with the correct info when asked, but most renters don't realize or know that they should ask about this. In addition, what is said (or not said) at the rental car company counter never supersedes the "fine print" of the actual contract that they make you sign.
Two other rental-car-restricted roads for tourists on the Big Island to watch out for:
The access road that goes partway to Makalawena Beach (between mile markers 88 and 89) north of Kona -- it can't be done with a regular car, but is feasible with a 4x4. So although Jeep renters assume it's OK to go, it's still under the restrictions for unpaved roads.
The Government Beach Road in the Puna district (between the subdivisions) -- this one was upgraded with new gravel and grading, when lava threatened the closure of Highway 130 a couple of years ago, but the restrictions still apply.
Again, the exception to most of these restrictions are with Harpers' 4x4s. Over the years, I have not chosen to rent from them myself.
Caveat emptor for all car rental companies!
Here's another detail that most renters don't realize, should they be in an accident on a rental-contract restricted road:
Many local drivers on the Big Island are still (or again) either uninsured or underinsured, despite Hawaii being a "no-fault insurance" state. Other visitors doing the same in a rental car have also voided their own insurance, if they have broken their contract as well.
So although you may be a very skillful driver, others may not be, and your level of financial risk is higher than you think.