0ldsch00l :
Thank you for the copyright law definition, but I think I know that already. But if I paint a new picasso and sell it does it mean the guy hanging it in on his wall created it?
That's not really a good comparison though. Obviously no one is going to claim that the original developer of a game series did not create that series. Again, if the developer sold the rights to make new games in the series, then they already got what they wanted out of the deal. If they decide otherwise later on, then they probably should have negotiated a better deal, or held on to the property for themselves.
In your painting scenario, once you've sold that painting to the guy, he can do what he wants with it, within the bounds of what was agreed to by the sale. If its value were to somehow climb to ten times what he originally paid you for it, he can sell it to someone else, and doesn't owe you anything. You already received the payment you agreed to.
Perhaps a better comparison would be an artist designing a logo for a company to use, and taking a one-time payment for that design. The whole point of the agreement would be that the artist is trading away their rights to make money off that particular logo in the future. If they didn't agree to letting the company do what they want with the logo, then they wouldn't get paid. They're willfully making an exchange, for the benefit of both parties.