If a keychain has been unlocked, people using your computer can see your passwords anyway with something like security find-generic-password -l AppleID -w.

It's an additional security risk if it would allow scripts or browser extensions to see them as well. Could you write a script that opens https://accounts.google.com/Logout (which redirects to a login page), modifies the password field, and copies the password?

The point of the warning is twofold:
1. be aware that your passwords are not safe, even if all you see is ••••••••
2. entering commands in the terminal and interpreting the output is for a determined few, who don't need this, indeed. But careless use of this simple trick will show your passwords to the entire world, pranksters included.