There is a market for English-language textbooks in non-English-speaking countries, in particular in engineering; that one doesn't surprise me at all. My minors when studying in Germany were computer science and mathematics, and at least half of the textbooks were in English. Occasionally I see English-language textbooks on bookshelves here in Kyrgyzstan, for what it's worth.
What the publisher did was probably that there was demand for their English textbooks by Thai students in Thailand, and they could either profit from that or leave the market to pirated copies. So they took the camera-ready files they already had for their US editions, took them to a printer in Thailand (where printing is much cheaper than in the US) and had a local print run made. This cost the publisher basically no money other than the paper and printing, so they could make a profit in the local market even at a much lower price than in the US. Note that the local market needs to be distinct from the US market for this to work.
The whole pricing model is basically dependent on a clear price distinction for the same product between a "core market" (Western countries), upon which the publisher's production process is based, and secondary, low-price markets (Thailand in this case), which basically works as a byproduct of reusing their production from the core market. Of course this business model breaks down as soon as someone re-exports the products from the secondary into the core market.