What actual law did they break? Nothing I've read said it was illegal. They didn't truly tamper with the actual product. Not only that but there is a PENDING recall that's being looked at so they will probably re-release with additional content or something to make up for the voucher that was removed. Don't count Gamestop out until after the fact.
"As part of Deus Ex: Human Revolution's boxed offering on PC, Square Enix included a third party coupon. GameStop was not made aware of this inclusion and Square Enix respects the right of GameStop to have final say over the contents of products it sells and to adjust them where they see fit in accordance with their policies." -Latest comment from Square Enix.
[Edited by fire_foxxx, 8/25/2011 12:42:06 AM]
Sales of Goods Act for one, and possibly a few under something similar to the Trading Standards Act in the US.
Like I have said, Square can put whatever they wish in the packaging and it is not up to Gamestop to decide to remove it, they have opened a product and removed an item that is included with the full packaging, thusly not giving the customer the choice if they wanted to use the enclosed key or not.
Square has been so far willing to help gamestop, but they also did not have to be so willing.
[Edited by DABhand, 8/25/2011 5:52:44 AM]