Try explaining that perspective to your spouse after you lose money in a 419 scam. "But honey, I didn't know that this Nigerian was a con-artist. I had to give him a chance!"
You know, this is exactly the same reasoning (prejudice on the basis of generalizing a correlation between a behaviour and a shared attribute) as exhibited by racists. In the same way as it is not fair to discriminate against black people on the basis of a higher relative proportion of criminal convictions, it is similarly not fair to discriminate on the basis of someone being a Nigerian in Nigeria on the basis of 419 scam incidence.
That's not to say it may not be effective, on average, in limiting losses. But effectiveness is something distinct from fairness.
