That would have been my response, too. Why would they care what he was asking? Doesn't he have a right to know why they want to know? What harm was his question?
That's not the way authority works. You answer the policeman's question first then he answers yours. There is nothing new about that. It was like that long before the WOsD's and WAY before the Patriot Act. The harm was that he made himself look suspicious by being uncooperative with a perfectly reasonable question. If his rights have been violated he can take it to court. But you know he won't because he doesn't have a case, does he?