The cookie law is not implemented as a browser function but something that operators need to implement if they want to legally track users. It's an annoyance because each implementation is different and every website wants to track users. If EU went after Web browsers and made them implement the legislation as an API, we would have had tracking prompts like location or camera access prompts and probably kill the tracking industry in a similar way Apple killed App tracking with their tracking prompt.

In the case of UIWebView/WKWebView (AKA the browser within an app that can access web data), this can be implemented by Apple as any other data access prompt like location data access or App tracking access for example.

Apps like to track user data like user location too but thanks to Apple's implementation of prompting the user first, they need to have a legitimate reason to request that information.

So, I guess, apps can claim that they need to access web data to provide some service(like widget, sign in session to transfer the login into the app etc) and users who want that can accept the requests and those who don't can have peace of mind.