Yes, but if you write your own extension maybe you want to read and modify the data. For example, patch fingerprinting script so that it gets the wrong result.
Furthermore, malicious extension can read the data from the DOM, from forms (for example, password or credit card fields), and in some cases, from JS variables. They can insert fake information into the page. So preventing extensions from reading network data still leaves a lot of options for a malicious extension.
Furthermore, malicious extension can read the data from the DOM, from forms (for example, password or credit card fields), and in some cases, from JS variables. They can insert fake information into the page. So preventing extensions from reading network data still leaves a lot of options for a malicious extension.