My guess would be that it's policy-enforced and what Apple really has problems with is not browsers as a concept (dynamically loading and rendering data) but rather the execution engine that websites usually use (JavaScript) and seeing that as a too-large-to-review area where they want to avoid having vulnerabilities in applications published on App Store.

Or it's a anti-competitive move disguised as a "avoid vulnerabilities" but in reality they just really want to make sure iOS users keep using the one and only browser (Safari).