If my buddy sells me a used game for $40, he can no longer play it online, as he lost the online portion in the transaction along with the rest of the game. However, if I need to buy an online pass, then I didn't receive the online portion. Effectively, part of the game simply disappeared in the process of a used game sale. Or, more accurately, the publisher basically stole it back to resell it.
As I see it, it's highly dishonest and greedy to demand extra money if the game's already been paid for. They have already received their money for a single copy of the game; it's not like a used sale creates a second copy of the product. They have no right to demand payment for something that has already been paid for.
Imagine, as an example, if every television were uniquely identified in a way that every set top device could recognize. Imagine further that you have a DVD player and a movie. You decide to resell the movie because it was Disaster Movie, you regret purchasing it in the first place, and are hoping to cut your losses. Should the person who buys it after you be barred from watching the special features since he didn't buy new? Does the fact that the studio isn't seeing ADDITIONAL profits from an ALREADY-SOLD unit mean that he is not entitled to everything that normally comes with it?
What about physical products? If someone resells a bookcase, does Ikea have the right to take the shelves back and demand payment so that the customer can have what they already purchased? Ignoring the fact that such a scheme would obviously be unfeasible to implement and impossible to enforce, it would also be a slap in the face of consumers' rights, and in most places considered illegal.
If a person sells a game and the buyer doesn't receive all of it because it's used, the publisher or manufacturer is STEALING. THIS IS THEFT, PLAIN AND SIMPLE. The entire game has already been paid for, and there are no additional copies in circulation as a result of this sale, so the publisher has no right to demand extra revenue from it.
P.S. Thanks