I don't think that this specific case is that clear. Textbooks, which admittedly sometimes are quite expensive when you buy them in a 1st world-country, are indeed often sold for much less in development countries.
But I think the publishers do some mixed calculation here; selling the book for the lower price in the entire world wouldn't be cost-covering, especially for elaborately made books , e. g. with many figures. However, to print some more and sell it for a lesser price in 3rd world countries makes those books affordable for students there and probably reduces printing costs per book, and therefore perhaps also makes books cheaper that are sold in the USA or Europe.
At least with books frome the UK there are sometimes even special versions, eg with paperback instead of hard cover, that are subsidised by the british government as a development aid (maybe there is some equivalent subsidy from the USA too, I don't know),. On most books on sale in 3rd world bookstores I saw, it is explicitly stated on the frontispiece or inside, that those books are "only to be sold outside the USA," or "not to be sold in UK".
Hence, I think the situation is different here from selling a used car on ebay, and I cannot fully approve this business model, especially in case it involved reselling of subsidised books.
I must admit that I myself took advantage of those cheap offers when, as a medical student, I was abroad several times for practical training and internships in Egypt, Sudan and Kenya, where you could get e.g. "Cecil's Textbook of Medicine" for merely 20% of the US price.. However, I only bought those books for personal use, not for resale.