...and if your profit margin is close to that 30%, as it is with many businesses, you are now required (should you choose to develop an app for iOS) to sell your product to users whom you have to support and stand a very good chance of gaining nothing from.
In other words, such a business would be required to offer an option to hand off all of its profit to Apple, should it choose to offer a native iOS app. Apple gets a better experience for its customers, and the third party gets more customers it gets zero profit from. I can understand if such a business would find this unacceptable. (And look at it from a user's perspective: if given two options, one-click billing through Apple or time-consuming credit card entry through a web portal, which do you suppose the majority of users will take?)
So how exactly is that understanding flawed?
Edit: More details added, and some wording correction.