Because the whole Web movement is trying to make browsers into the OS.

Already done; see Chrome OS, WebOS, Firefox OS, probably others.

I got you, but those are actual operating systems. Regular browsers should keep being regular browsers, limits respected...

This clipboard thing remembers me of the webrtc functionality that enable browsers to scan my network without asking me.

Related: https://news.ycombinator.com/item?id=11407536