It actually creates an interesting legal conundrum. In several countries (at least here in mine), the customer has the right to see anything he agrees to spelled out to him before he does the purchase. A link on the box is not sufficient in this case because it requires the user to have an active internet connection at the moment of purchase, which cannot be taken for granted. Hence, the user is not necessarily bound by the this license, because he couldn't see it before he purchased the product. Theoretically, the vendor must take the product back under these circumstances.
However, if the user already installed the game (clicking the Steam license away because he knows that it isn't valid under these circumstances), then he agreed to a contract under American right, concluded on an American server.
The customer could still bring the product back to his European vendor, claiming that he doesn't agree with the Steam license and that he couldn't view said license before the purchase. The vendor must then take the product back. However, the customer still has a contract with Steam, and Steam will not nullify the contract because from the perspective of Steam, under American right, the contract is still valid. This in turn means that the vendor can't resell the product since it can't be activated any more. Odd indeed.
Note: the above is just one possibility, there are others. It'll probably take a specialist lawyer for international licensing and customer rights to determine which one's correct. Actually I think there may be a gray area here.