Why do Windows & OSX allow apps to grab content from clipboard without restriction?

I recently made a horrifying discovery while writing my own first program that accesses the clipboard.

I always took for granted that clipboard-paste worked like a message-delivery mechanism... you copied something into the clipboard, then the act of hitting ctrl-v delivered it *only* to the app running in the window with input focus (say, by having an app decide to make itself capable of receiving such clips by calling a system method like setClipboardListener(this) or something similar).

Spoiler: Windows and …

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