Jim,
I would hazard to say that most Americas are no more worried about being questioned than are Canadians. Threads like this tend to play to the extremes.
I've been shooting cameras here in the US for almost as long as you are old. I've never been stopped, questioned or challenged in doing so anywhere.
Admittedly I use discretion. For instance, my nephew is a US Marine helicopter pilot. We just visited with him last week before he was deployed to Iraq. He took us onto Camp Pendleton to show us the stuff he flies.
I had a camera with me and asked him if it was alright to take pics. He said there were no restrictions and I did so. Sometimes its wise to be the first to ask the questions.
As I said in my first post here - I was not with the OP and cannot know what occurred. I can only know what he told us. There are at least two sides to every story - so we have, at best, only half of the info we need to make a determination of whether what purportedly occurred was reasonable or out-of-line.
But at the end of the day, I don't think most Americans are more worried about be questioned than you, as a Canadian, are. The vast majority of the people here also think the system is inherently fair and recognize that members of law enforcement are here to protect us.
We are, perhaps, more vocal as to our individual rights than Canadians because of how our nation came to be (i.e. via rebellion from the prior authority) and how we early on enshrined our individual rights into our Constitution. But that vocal display should be viewed as a strength, not a weakness.