Why are some (international study) books ';Not for Sale in the United States';?

Study books like the 'International Student Edition of Calculus Early Transcendentals' by James Stewart (a standard university mathematics book, www.stewartcalculus.com) explicitly state that they are ';Not for Sale in the United States';.





I am wondering for what reason that is.





Has it anything to do with the price, or perhaps with the inport/export of knowledge?Why are some (international study) books ';Not for Sale in the United States';?
It's all about price.





The international version is usually paperback (vs. the US edition will be hardbound), but otherwise the content is the same.





It's the same for Movies, CDs, and other media. The US version is always more expensive for a variety of reasons, but import/export of knowlege has nothing to do with it.





Publishers know Americans can pay more (based on percentage of income), thus we are charged more. It's exactly the same as presciption drugs.

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