Earlier quoted context omitted.
You're looking at it wrong. The 30% store fee was a known cost of doing business before anyone spent money building the product. That cost would have been built into a determination of whether they'll need to set a retail price of $9 or $7.
But the point is that you might have a viable business at a price point of $7 that you won’t have at $10 (with the extra $3 all going to Apple).
Or perhaps their business wouldn't be viable if iOS and Android were Windows-esque free-for-alls with rampant spyware and malware, making a good portion of their potential user base wary of installing apps.
Or perhaps their business wouldn't be viable if people haven't become accustomed to spending money in App stores without worrying about credit card fraud, etc.
Or perhaps their business wouldn't be viable if iOS and Android had no mechanisms to protect against app piracy, and 30% of their potential customers pirated the app.