1. Because despite popular belief racism colorism, and thinking that Blackness is bad isn't limited to just the USA.
2. Some people can't seem to grasp the difference between race and ethnicity. Some people think Black means American. It's not just Dominicans, many others do the same thing. I've seen other Blacks say Zoe Saldana's not Black. I've seen some people try to say that Nicki and Rihanna aren't Black, "they're Trinidadian and Bajan". It's interesting because nobody seems to feel that way about any other race. There's white celebs from England, France, Australia, Canada, etc. and nobody says they're not white.