Why did a guy say to me "Welcome to America" when I came from the Dominican Republic?

Yes, arent you clever, you must be the first one to figure out that this little conundrum exists. Its because the Dominican Republic is not officially named the Dominican Republic of America. Brazil is not Brazil America. But it IS the United States of America. Its a part of the name of our country. Thats why it is said that way, and to ignore something so obvious is just being obstinant.