Because they have different designs on the back, a non-standard border color, and are also considered marked because of the autograph printed on the front of the card. The most they can be used for is play-testing with friends.
True but that really doesn't sit well with me, to use the same card to beat a card. We follow the rules on the cards and the standard rules (poisons, card layout etc). If all the difference is a card backing then that isn't a problem for how we play. :-)
Imagine how much money Nintendo would lose if those decks were tournament legal ? The secondary market of cards would just plummet and nobody would buy products except for the Worlds decks to pick up very cheap EXs,trainers etc.
They are designed for casual players who do not have the money to spend on booster boxes, collectors tins and booster packs. They would rather wait till a "Worlds 2012" deck is released and then be able to play casually with friends without being completely destroyed.
Some people may just buy them as they are a fan of the player who won or topped an event with that particular deck choice.
There are many Pokemon Masters that are popular in the community currently such as all the guys at "The Top Cut" who have a YouTube channel dedicated to showing deck reviews and Pokemon card battles at events !
Thanks. I was merely curious as to whether the reason they were illegal in Tournaments was because they were badly designed and made too powerful. Casual playing and homemade tournaments are the best because they allow freedom. As I said earlier, if the card is not allowed ONLY because of its back then that is all good with me. I can cover the back by sleeving it. :-D