>What next, should apple take a 30% cut of my rent because I found my apartment on the Craigslist app? Should they take a 30% of my train ticket that I purchased using the Safari app?

Sure they could, and usage of those products to purchase goods would nominally drop to 0%. People do not care about a lot of things, but they do care about losing money.

Apple would then force the with-IAP price to be the same as the without-IAP price so that they get a 30% cut of your rent regardless. You may be underestimating their willingness to tax all economic activity