Google Docs did this for a specific reason. Browsers restrict access to the clipboard on general user actions (like click, right click, etc). On the other hand, if you use ctrl+C or ctrl+V, the website gets access to the clipboard, because it's a JS clipboard event. So those work in Docs. Think about it - do you want any website to just read everything off your clipboard?

Docs overrides right click because there's things you'd naturally want to do in a word processor with right click (format, link, comment, etc). Because docs shows a custom menu on right click, the browser will not expose the default menu, and without the default menu, there's no JS copy/paste event.

Browser extensions have more access, and that's why a browser extension re-enables the functionality.

So it was a design choice: the Docs team decided that giving users all these other right click actions was more important than allowing the user to right click to copy/paste without an extension (since most people use the keyboard shortcuts anyway).

Good points, though, I don't agree with "most people use the keyboard shortcuts anyway".

Most not-so-tech-savvy users I know, don't know keyboard shortcuts, find them hard to remember, clumsy and slow (at least until they learn how to really use them).

I think that googles decision was valid and the best compromise they could have made. I really don't see a way how they could achieve both, without compromising the user's privacy in a manner that doesn't require a specific browser or extension.

Most people, actually, use the toolbars. Neither right-click nor keyboard shortcuts come close to toolbar usage amongst "normals"

The toolbar works just like that. It tells you to press command + V (or control + V on Windows) as well.