Because it fits. Do you think that Wal-Mart doesn't have rules for vendors?
Perhaps Apple is more mercurial and arbitrary. But the institutional arrogance is the same.
As a manufacturer, Wal-Mart is your best friend and worst enemy. They pay you promptly and order lots of stuff. But they demand steep discounts and punish you harshly if you fail to meet commitments, and you must be able to rapidly ramp up your supply chain when their demand grows.
Your analogy only address's the issue form the consumers point of view.
What about if you make a product, should you be able to force a distributor to carry your product?
It functions as such. Developers make a product, they convince Apple to stock that product, and Apple takes a cut when they sell it. You can argue that it shouldn't be like that, but you can't really argue that it ISN'T like that.
The only thing you can really argue is that they should allow other app stores to exist that aren't under their control. And frankly, they do exist on jailbroken phones and if you want that you can jailbreak your phone, or get an Android.