I don't think that follows. Allowing applications to intercept and mess with other application's network traffic would be an obvious security issue; that's far more likely to be the reason than some kind of vague "we want to track your data" thing.
It's certainly possible that Apple could construct an appropriate API for allowing users to configure apps in such a fashion that they could monitor network traffic, much the same way as similar APIs exist for accessing e.g. photos. But since it's a niche application at best, I'm hardly surprised they haven't done so.