Why would Papa John's tax the delivery fee on my order?

I recently had a pizza delivered from Papa John's and I noticed on the receipt that the $1.50 delivery charge had been included in the amount that was taxed. So say, for example, that the purchase was $10 and the delivery fee was $1.50. I was taxed for $11.50 instead of $10. I was always under the impression that they could only tax the purchase, that the delivery fee was something different. Am I wrong? Why would they do that?