My takeaway lesson is that the permissions model for extensions is confusing and nearly useless.
[1] https://chrome.google.com/webstore/detail/obscura/nhlkgnilpm...
I could see distinguishing between extensions that in any way exfiltrate data from the pages you view, versus extensions that process the DOM and do something locally, but never send the data anywhere.
This requires a bit closer vetting than Google currently does, I think. To demonstrate that all processing happens locally, we encourage our users to load various websites with our extension toggled off, then go into airplane mode, and then turn our extension on. This doesn't strictly guarantee that we're not separately exfiltrating data (we aren't), but it does prove that our core process happens locally.
In short, if there were separate "read" and "write" permissions, I would only need "write". For privacy-concerned people, that's a very important distinction.
For example, a web clipper operates on multiple domains, but it can avoid it by using activetab permission instead and then offering optional permissions if it wants when you click on the clipper extension icon.
If you want something to be done automatically on multiple domains, this is not possible without that permission. Not unless you want to annoy users with prompts.