The option is labeled "cancel". There is no warning this will be considered a deposit.
The word cancel is pretty clear:
1a : to destroy the force, effectiveness, or validity of : annul <cancel a magazine subscription> <a canceled check>
1d : to call off usually without expectation of conducting or performing at a later time <cancel a football game>
2a : to mark or strike out for deletion
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http://www.merriam-webster.com/dictionary/cancelThe problem, I suppose, is that once issued the code becomes a negotiable bearer instrument. And the issuer (Mt. Gox) has no way to tell that it code wasn't used in trade (e.g., for use as escrow) where perhaps following the trade you redeem it yourself.
So I can see why they might have this policy. But I can't see why they would label the action "cancel" when clearly it is not cancelling anything. Instead they are providing a method making it easy for you to redeem one of the codes you had created and nobody else had redeemed. It is like if I borrow money from you -- me repaying early would not mean that I had canceled the loan.