When you install software from JetBrains and have the registration key on your clipboard it will autofill it into the key input field.

There are so many programs using this that it all boils down to trust.

Personally I don't trust LinkedIn so I keep away from them. But in this case they still might have a valid reason for this.

The fix: be more transparent about why the app is doing it. And the browser/OS could show a popup every time an app reads the clipboard to make the user aware of this.