Earlier this year, Avatar was released on DVD and Blu-Ray. The release was as bareboned as it could be, as it only contained the movie. In a few days, Avatar is going to be released again, but as an extended collector's edition version. It will contain all the expected extras that should have been on the first release. For people that have already purchased Avatar, and want this special edition, they will end up with two copies of Avatar, the first of which will be completely outdated, and will have minimal resale value.
Nobody is forcing anybody to buy an extra copy of the movie, but it is the underlying motive on the part of the movie studio that is in question. The studio deliberately withheld the extras during the first release, so they could get customers to buy the movie a second time. Though James Cameron may be attempting to sell people the same product twice, he is far from the inventor of such marketing. This same approach to marketing has been done before, as evidenced by other movies, books, and video games.
The real question is not why corporations are allowed to double dip into their customers' pockets, but why customers allow this to happen. All it really comes down to, is customers being aware of such practices, learning to be patient enough to wait it out for the second release, and not bite into the early three day sale bait. Corporations will never stop being greedy, and they will continue to be this way, as long as customers allow it to happen.