I can say from first hand experience that well meaning individuals have rather poor abilities in predicting whether someone is a U.S. citizen or not. Over the years, so many times it’s been assumed I am from India or I've been asked where am I from (as in what country) vs. white friends, some even born in Europe, who have not been asked. I was born in the U.S. I received a Princeton bachelor’s degree and double-major in electrical engineering and the Woodrow Wilson School of Public and International affairs and a Stanford master’s degree in electrical engineering. My family has spoken English as its first language for generations. I have no accent, am an Eagle Scout, am athletic, pay my federal and state taxes, tithe to my church and community, volunteer as a Eucharistic minister at Stanford Hospital, look at people right in the eye, and otherwise am as "all-American" as the next person, not that any of these qualities define an American citizen. A logical hypothesis might be that I'm asked because my skin color isn't white. At work, I have witnessed, once as the employee and once as a supervisor, where whether a person was white or not directly influenced whether they were asked if they needed or were inferred to need an export license – something required only for foreign nationals of certain countries. I took corrective action, but as you can see, even trained managers, erroneously relied on physical appearance or race.
In case it's not clear, I am not particularly worried about how SB1070 will affect me, but I believe my experiences are illustrative of the unconscious bias, which is as harmful as intended bias, that Hispanics may face if Mexican appearance, language, or customs are allowed as factors in determining reasonable suspicion. I believe it's important to do the right thing even if you or I, based on where we live or our ancestry, are unlikely to be impacted.