Would even someone prejudiced enough to be automatically suspicious of Muslim immigrants necessarily suspect a couple of ordinary looking Caucasian guys (the Tsarnaev brothers) as bomb planters?
What about an ordinary looking white guy parking a rental truck in front of the BATF office?
“See something, say something” is definitely useful. Indeed, effective law enforcement depends on it. But it does give bigots a way to cause targets of their bigotry to be hassled by police (or airplane security, etc.).