It often has to do with author rights. If the rights that an author sold to a publisher are US only, then the publisher only has the legal power to sell withiin the US. Usually the rights, though, are North American, and then foreign rights can be sold individually or as "world" rights.
Some retailers don't always abide by those rights and do sell to customers outside the legally sanctioned area. It would probably be up to the author to constrain that.
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