I vaguely remember that there used to be excise fees (i.e. U. S. government taxes) on frequent flyer miles awarded, and before the 50 cents per day / $2 max per rental b.s. Hertz just passed this fee through onto the rental receipt.

Yep. I found more information on it via Google.

In Hertz's case, that smallish excise tax didn't last long once it was implemented, as it was replaced with the current, much higher charge.