What is Apple's rationale for saying that apps can not charge within themselves? The intention is to make more money, but what is the stated reason? Couldn't Ap

What is Apple's rationale for saying that apps can not charge within themselves? The intention is to make more money, but what is the stated reason? Couldn't Apple make plenty by charging up-front to install things, for the "utility" of it's marketplace and userbase? I struggle understanding why Apple should get every right to money exchanged within apps they didn't program or administer.