Some apps choose to make their apps free, others choose to charge fees. Some want to have their in-app purchases charged 30%, others don't.
Lots of products charge you an upfront fee, and then more fees for various things in addition. I don't think you've got much of a leg to stand on with that argument. That would be like saying, I paid for my cell phone, I don't think that cellular carriers should charge me again for the monthly service -- they should bundle it all one.
It's a design and pricing choice. And you buy in or you don't.