Apple doesn't charge for person to person transfers, or buying physical goods. They only charge if you are selling digital products (apps, features, content) through the App Store. Just those things that cost Apple to store and transfer.
Do you really think WeChat pays Apple for the file transfers for hundreds of millions of copies of WeChat to customer iPhones every month (when you include updates). Essentially at least 100 Megabytes times 100 to 200 million a month.
It's entirely relevant. That 30% pays the costs of the app store, and bandwidth is probably the biggest cost of running the app store.
Please show me where Apple says the $99 I pay every year pays for their Sales, Marketing, Distribution and Operating costs of running the app store and getting me customers.