it seems like a perversion of my beautiful REST/JSON server

My UIs are data/store driven. The UI is just a way to visualize the data. Your data could flow through all of of the extensions and the extensions can make decisions (e.g. setting visible to false). Like middlewares in a Connect/Express/Polka app. And the UI doesn't even know about all this, it just updates with the current state and makes sure it's consistent.

This is done to separate internal representations of information from the ways information is presented to and accepted from the user.

React is an isomorphic/universal framework. That means that there is a virtual representation of the UI component tree, and that is separate from the actual rendering that it outputs in the browser.

As a result, web browsers can provide only minimal assistance to humans in parsing and processing web pages: browsers only see presentation information.