Correlation does not imply causation.
Ethically, I do not think it is correct to outright flag an entire country. It can be used as one factor, but that in itself is not enough.
Japan for example, a country that is not usually associated with crime, has one of the most profitable crime organizations in the world, the Yakuza. The UK might be where most transactions tied to money laundering are conducted, and so on so forth. In the grand scale of things, Nigerians are in a lower order of magnitude when it comes to dirty money.
Fraud detection, at scale, takes not one or two but many factors in consideration. That would produce a level of confidence that will translate into a suggested action. If the confidence is high, a fraud detection system will take an action without human intervention. If the confidence is low, a human needs to intervene and take action.