Making a leap from circumstantial evidence to a conclusion on the basis of ethnicity
Nigerian's not an ethnicity, it's a nationality. Having said that, I work with enough Nigerians to know my Yoruba from my Igbo and can generally tell by someone's name where they're from, just as people do with , say, Orkney or Basque names. "Cultural awareness" to some, "racism" to others.
I think British retailers before Chip and Pin were much more strict about checking signatures on strips as well though.
Only up to certain amounts. When I got my card stolen about 20 years ago, I found out from the police that the bar was set at £40 before retailers asked for ID and made a phone check. Thieves knew that too.