EA is not at fault for listing on the cover what comes with the game when you buy it NEW,
Except that EA does not say that it comes with the game when you buy it new, they just say it comes with game.
which is intended to be bought NEW since that is the only way they sell it.
I can't say whether that on specific employee said the DLC came free with the used copy, but the company does not (and never has) said that. GameStop, as a company, is not even slightly to blame here.
You wouldn't know that as you weren't there and don't know the questions the customer asked the employee. GS employees are known of being very misleading (MH3 demo disc ring any bells - the company was telling employees to force a $5 preorder on a free demo disc) and just generally uninformed, so they may just answer the question in the way the customer wants to hear (which happens all the time in sales, especially retail).
Gamestop is the one to blame (with some small portion to fall on uninformed consumer) since they made the choice to buy back and sell games used. EA relinquishes all fault once the original sale is made, the game is opened and included content is accounted for (whether it's used or not). It really is as simple as that.
GS could avoid it by informing the sales clerk to inform the customer, put up a sign that says all used sales may or may not include DLC or extras from original purchase, or just put in a false cover that GS can print themselves that looks identical to the original cover, just doesn't mention any of the DLC and extra goodies(it can even have a GS logo on it somewhere). That should've been part of their business model from the get-go.
EA doesn't sell used and therefore shouldn't have to plan for used games since they never intend for their games to be sold used.
If EA are doing this in response to making no money from used games, then don't sell new games. It's just a way to extract additional revenue because they don't see any of it and they want in on the party.
Making money from used game sales may be part of it, but I'm sure it's more about making online a self-sustaining feature since servers cost money to run and maintain and if people are gonna continually extend the life of the online portion of a game through used sales to late-comers, then those late-comers should bear part of the burden of running the server, just like the original purchasers did when they bought the game new.
It's business. You can't expect EA to foot the bill for servers out of their own pocket forever just because we want them to. Gaming is a serious and very expensive business and they have to find a way to keep making a profit otherwise you don't end up with new games with new features, better graphics, new gameplay and lagless, seamless online with dedicated servers and always someone to play with across all systems regardless of time of day.
It all cost money and it has to come from somewhere.