As far as I can tell, textbook publishers practice price discrimination so that they can sell less expensive versions of the same book in markets where profit is maximized at a different price. Retailers are only permitted to sell the appropriate version in the appropriate market.
Personally I have seen many textbooks available in the US/Canada version and Indian Subcontinent version (generally the same text, often for sale at <25% the price). My guess is that it's common to see India in particular because there is a large market for English-language textbooks there.
It falls along the same line of argument that big Journal Publishing groups like Elsevier and Springer do the same when giving subscriptions to countries across Europe vs Africa. I heard this in a talk from a former director of Radboud University in Netherlands who is now spearheading the Open Science movement in Europe.
Personally I have seen many textbooks available in the US/Canada version and Indian Subcontinent version (generally the same text, often for sale at <25% the price). My guess is that it's common to see India in particular because there is a large market for English-language textbooks there.