> The point is, when the user copies a piece of information he has a clear target for that information (clipboard is the tool he uses do copy from one window to another), why would all other apps have access to it?

Everything that is run by the user runs in the same context. There is little point in focusing on the clipboard, because any two programs running within the same user can do literally anything to each other. Including spying on each other keystrokes, modifying each other's text widgets contents, and faking whatever text/images they are displaying. The fact that they may alter the clipboard contents is irrelevant when they can literally monitor and inject keystrokes on each other.

This is the security model that we have right now at the desktop. Good luck trying to find a better one that just does not completely break UNIX and turns it into something like Android. One of the benefit of this model is that you (the user) have practically unlimited freedom in hooking the system in ways that nobody expects (e.g. a clipboard access detector), but then so does anybody else you decide to trust (by running their program in the same context).