It seems there is currently no pure JavaScript method for accessing the system clipboard using most modern browsers, Internet Explorer being an exception. On numerous other Stack Overflow questions (e.g., Clipboard access using Javascript - sans Flash?) it's explained that this limitation is a deliberate security measure to protect against web sites reading passwords or other sensitive data from the clipboard.
While it seems obvious that reading from the clipboard would be a huge security risk, it's not clear to me why writing to the clipboard would be. What scenario, if any, are browsers protecting against by denying JS the ability to copy data to the clipboard?