I imagine they're trying to prevent the reverse, purchasing a cheap subscription in a developing country to use when you actually live in a more expensive country.
They know I live in the UK though, I have a UK-based history with Amazon going back 10+ years.
I believe they need a license to stream in different countries. If they don’t have a license to stream in Thailand I don’t think they will let you watch there.
Regardless, it certainly "feels" unfair.
I did try using a UK-based IP too, but that didn't work (it was a data centre IP, so I guess they block them all).