Retail stores don't have to carry items they they don't want to because those items can be sold at other retail stores. With the App Store, an ios app cannot be sold anywhere else. So if Apple can just determine arbitrarily what apps can and can't be sold, they can effectively destroy the software companies because there is no other market (IOS apps will only work on iOS, and can only be bought from the App Store.) That is extraordinary power, and frankly too much power. I don't get why some people can't see this.
The difference is that an app is an extension of ios. It's not just a "product". The very reason "an ios app cannot be sold anywhere else" is because it has no independent existence-- it is a meaningless entity outside of ios.
Now, one could argue the opposite-- that an ios app could be ported to Android, thus having a distinct existence. But in this case, the app CAN be sold somewhere else.
BTW, this is exactly the same line of reasoning argument that a woman should have the legal right to abort a fetus she is carrying. So if you were to favor legalized abortion, you should certainly be siding with Apple on this. (The converse doesn't work precisely, because someone may object to "murdering" a fetus on moral grounds that wouldn't extend to a piece of software).