However, if you use part of the Google Docs UI to trigger the copy/paste operations, then the javascript code which runs the UI has to have access to the host OS’s clipboard, which it wouldn’t normally have. Otherwise it would have to use its own non-OS clipboard, and you would only be able to paste back into Google Docs and not into other applications – and then if you pasted, it wouldn’t be immediately obvious whether it should paste the local Google Docs clipboard or the OS clipboard. Confusing.

They could have implemented a workaround such as duplicating the content selection into a visible user-selectable area, selecting it (which you can do in javascript), then triggering a client-side copy operation (ditto) to get the data on to the system clipboard. I’ve done this on some hacky code in the past. But it’s ugly as heck, so instead they’ve gone for a slightly less ugly option: requiring a browser plugin.