What does this have to do with copy and paste?

Copy and paste is a form of IPC. IPC needs to be secure, or (see above).

If you don't trust the free game you just downloaded, you might not want it to be able to see what's on the clipboard. Some number of people will copy their passwords, credit card numbers, etc. If an app phoned home the contents of the clipboard every time it ran, eventually it would pick up some private information.

And that's assuming that all the bug gives you is the cut buffer. Who knows how they were actually led to implement the feature?