For example, you speak of "presumption of innocence" and "burden of proof" as "core concepts of our justice system". These are only core concepts of our criminal justice system; our civil justice system has very different rules for burden of proof, for example.
Some countries have much, much higher percentage of fraudulent transactions than other countries, and it's perfectly reasonable for Western Union to use country of origin as a factor in raising the fraud red flag. Western Union doesn't have the power to arrest anybody, and they have no reason to "presume innocence" for any one of their customers. If anything, when it comes to monetary transfers, I think it's safer to "presume guilt", that is assume all transactions have reason to be fraudulent, and then only let transactions through if you have strong reason to believe they are not fraudulent.