Copy function is normal.
Paste function is not.
Either use the Copy and Paste tool by left clicking on the timestamp associated with (directly above) the comment you want copied and then left clicking on "Copy and Paste". This will copy and paste the entire comment into the message box.
-OR-
Copy any comment as normal and then click in the "Enter Message" box and then press "Ctrl" + "V" on your keyboard.
Do not know why INNO does this; I was not around when they implemented it. However, that is their means to an end.