I don't know if it is the case for this book, but many text books have vastly cheaper versions that are printed in India. Usually they are on much thinner paper. (I saw these during postgrad when some of the international students would bring texts from their home country.)
Could they have been pirating the books? In my home country (Turkey), there were alternative sellers (basically photocopiers) who made pirate copies of books (usually on much thinner paper and washed out colors) and sold them. They were exactly like bookstores and they'd have the books you needed in stock. They usually sold textbooks for 1/10th of the price.
Pretty sure it wasn't piracy. For instance see this[0] book (not affiliated), at the bottom left it says "Restricted! For sale only in India, Bangladesh, Nepal, Pakistan, Sri Lanka & Bhutan"
This also happened. But there was a completely legal tier of regional editions, my guess is also that this note is there because the regional rights were transferred to someone else.
In South Africa, we also have different versions from the US and they are not pirated. They are usually called international versions. It does have to do with pricing here vs. the US, but they are still expensive for a student.
If I remember correctly. A lot of books actually print in India. Then are sent to other countries.