Those cheaper copies for non-US markets are what's typically called (at least in the textbook world) as "international" editions. They're not allowed to be sold on Amazon, as per Amazon's policies. It's a shame too, because textbooks are usually so very expensive.

And as noted elsewhere, they have different ISBNs than the US editions, so if a seller send and int'l edition when you ordered a US edition, they did not send the book you ordered and the ISBN discrepancy proves that, so you should return it and complain to Amazon.