Microsoft published a research paper on "Why Nigerian scammers say they're from Nigeria". Was that research paper a stereotype that painted Nigeria in a false and negative manner?
https://www.microsoft.com/en-us/research/wp-content/uploads/...
https://www.microsoft.com/en-us/research/wp-content/uploads/...
Those who've been in the game for a while, have the ability to collect money from any country the "victim" feels comfortable sending money to.
Also, about the intentional grammatical error. It's not intentional. Many of them aren't educated. In fact, most learn to use the computer and internet on the job.
To take advantage of the dissuasive effect of saying you are from Nigeria, you don't necessarily have to actually be in Nigeria.