Thank you! Here seems to be the list of those states:

If renting in Maryland, Massachusetts, Michigan, New York, South Carolina, Virginia, or West Virginia: Upon signing the Rental Agreement, Hertz provides primary liability protection. However, such protection is generally no more than the minimum limits required by individual state law.

Here is also something interesting:

If renting in California Hertz provides no liability protection under the terms of the Rental Agreement to the renter from claims of injury by others against you resulting from an accident. Your personal/business insurance may cover your liability. Exceptions: Hertz will provide primary liability protection up to the statutory minimum limits to international customers (driver's license indicates an address outside the USA) renting in California.

Do you think this Herz is providing primary liability coverage (they call it LIS ("Liability Insurance Suppliment")) for "foreign" renters in California out of the goodness of its heart or there is another law on that matter?

As for all the other states, you are right:

If renting in any other state in the U.S.A, Hertz' liability protection is secondary to any other insurance coverage available to you. If you do not have liability insurance and/or the limits of liability of the insurance coverage available to you are not sufficient to cover claims by others against you, and Hertz, as the vehicle owner, provides liability protection due to an accident, you will indemnify Hertz for any and all payments made.