It may still be unethical, my point is just that there are shades of gray here.
From my understanding, the coupon recipients knew that the coupon was supposed to only be for a single $20 discount. The only shade of grey in this case (as far as I am aware) is that there may be a user who used the code and unwittingly received a discount applied to multiple products. I believe that the majority of people in this case knew that it was unethical (and possibly illegal) but rationalized it by saying that EA deserved it.
But it shouldn't matter. Real loss isn't necessary for it to be a unethical (or worse, a crime).
(I would argue in terms of importance, ethics > crime)
Plagiarize a paper in college, you have caused no real loss but still been unethical. Say you knew the topic very well and could have done the work yourself, you just plagiarized because you were lazy to get around the whole you harmed yourself argument.
The same deprivation occurs here. EA's reward for publishing these games is reduced or removed because people acquired them when the "door was unlocked".
I do not know enough about formal ethics to express my point here, but I would look at http://en.wikipedia.org/wiki/Categorical_imperative under Perfect Duty to show how the concept of piracy doesn't hold up under the Categorical Imperative.
Like stonemetal alluded to, in a pretty esoteric sense you are harming yourself by bringing yourself into disrepute.. but that is just quibbling. Also I guess the 'scientific' method employed in marking papers is as a proof which you have not given. Though you may have done the groundwork it does not automatically follow that you are able to reliably produce the required results. You may also then be bringing the school into disrepute... but, probably not the central issue here.
I don't agree that EA's reward is diminished UNLESS people who would have otherwise bought these games did not (which I would then absolutely regard as stealing) and ASIDE from the very real argument about server time (which I would argue is a separate instance of theft).
I don't think the social contract argument holds much beyond the idea of patronage ie. I have a duty to support the content producer, but no such duty to allow him to profit. That is arbitrage, I may find it worth my while to allow it, but I have no duty to support it. In abstract Kant-ian terms (thanks for the link, jogged my memory of all those philosophy subjects I studied way back when) if all the world rejected arbitrage people would only make things that were really valued (in real terms, some over-production allows for innovation of course.. things are never so simple).
In fact, in the OP, he mentioned that on some boards people were justifying their actions by saying that they were taking back some of the money EA had taken from them over the years. This could be read as taking back the profits, or the arbitrage, which they no longer felt were justified given EA's continued mistreatment of their custom. (or, of course could be read as a petty way to make themselves feel ok about stealing).