It's stereotyping.  I had a similar experience with some people in the Chinese community in Toronto.  We had chinese street-people attending one of our shelter programs, and asked a Chinese community organization if they could help.  They said no because there weren't any Chinese street people.  We said we had names and their personal attestation that they were Chinese.  The community organization then said that we were mistaken, that these street people were really Vietnamese or Korean, since Chinese people in Canada did not produce street people....and I assume that that may be because of the stereotype you highlight.