Asinine Tax

I rented a car from Avis this weekend (Pontiac Vibe - nice styling, roomy interior, decent gas milage, a bit sluggish but my wife loved it) for a trip to New Jersey to see my ailing grandmother. I have my American Airlines frequent flyer information stored on Avis's computers so that I automatically pick up air miles.

Turns out those miles aren't free and whats more they are a taxable commodity to the tune of 7.5% Now, considering the heavy discounting that goes on in the airline industry, the constant price fluctuations, and the rampant use of miles for the purchase of items other than airfare, how in the heck do they calculate the cost basis of those miles and if I end up using them on a lower priced fare am I eligible for a rebate on the difference??

Of course, the charge is small enough that I am not liekly to put up a big fight - heck it was only $0.06 for a sunday reduced rate rental. However, I imagine this tax could add up quickly if applied to only 75% of rental cars.

I've never seen it before. Have any of you?