How does it defeat your security if other people want to look at network traffic on the device they purchased?

Because I don't want apps to be actually sniffing traffic whilst pretending to be something else.

You could solve this with OS-level permissions for this kind of thing, the same way Apple now handles cross-app tracking

Sniffing network traffic would not defeat anything, provided you are prompted about the necessary permissions.

The problem with those permission prompts is that many users blindly accept them. If a “sniff all network traffic” permission were allowed, it would take all of one day before Facebook et al release an update with it and start siphoning millions of users network traffic.

Apple does not want this.

Apple would probably also prefer it if I had to use an in-app purchase to pay my rent, but luckily we live in a nation of laws, and Apple doesn't get to decide everything based on unilateral self-interest.