Why would Mt. Gox's Open Transaction method still require six confirmations? When withdrawing using Mt. Gox, one of the checkboxes is marked "Open Transaction" and it shows that this requires "6 Confirmations".

This is used if you might be paying another Mt. Gox account but want it to go through the blockchain rather than as a transaction handled internally by Mt. Gox and thus no corresponding transaction in the blockchain is made.

Since the input for the transaction is coming from Mt. Gox's own wallet, there is no concern over double spending.  Why would Mt. Gox require six confirmations for these if the recipient is another Mt. Gox account?
I think it's as simple as MtGox waiting for 6 confirmations before accepting bitcoin deposits to your deposit address, no matter where they've come from.

If you want instant transfers, don't use "open transactions".

We can try to make up reasons to try to make this policy make sense, but ultimately they make the rules and we act accordingly.