This statement tells us a lot about Sheldon's character, of his disappointment in the way society groups people based on race and/or ethnicity. Keiko makes a similar statement in "Jamaican Ginger" when, at the end of the chapter, after the Black Elks Club has been raided and she had witnessed people she knows being taken away by the FBI, she turns on Henry:
Keiko halted and looked at Henry. She looked down at his button, the one his father made him wear. "You are Chinese, aren't you, Henry?"
He nodded, not knowing how to answer.
"That's fine. Be who you are," she said, turning away, a look of disappointment in her eyes. "But I'm an American."
Write about what you believe Keiko means when she says these things to Henry. Be thorough in your explanation.