It's the principle of the thing. If I want to spend my money on a boob app, why should Apple stop me? I'm a grown adult, and it's my money.
I see. And you say the same thing to the manager of Target because they don't sell [insert product name here] you want to buy? Apple isn't stopping you from buying a boob app, it's stopping you from buying a boob app in their store. If you want to buy one in the Android or M$ store, you are free to do so, provided they are sold there. There is no Gd given or Constitutional right to purchase boob apps in the iTMS store.