They taxed my service charge

I had Tom Yum Goong tonight in a Thai restaurant in Na'ama and noticed on the bill that they had charged me 12% service charge and then a 10% sales tax.

I knew that it was going to be that way up front because it stated on the menu that they would do so, but what I didn't know was that they were going to charge me tax on the food subtotal + the service charge!

What would you tax a service charge for? It's like charging sales tax on a tip that you leave for a waiter at an american place. I didn't make a big fuss about it though because it was 8 cents difference. I sort of want my 8 cents back now though.