I am not sure if my thoughts are age related (under 50), Nationality related (Canadian) or due to having been a retailer in the past (not fabric).
I would never consider taking a photo in any retail (private) establishment without first asking permission. I am working on my son's graduation quilt (yes, yes he graduated a year ago, but it is coming along) and was buying fabrics in a LQS. I was buying based on colour, but had not chosen a pattern yet.
The clerk showed me a quilt that was made with fabric and a pattern they sell, but they had modified the border. I asked if I could take a photo of the border, so I would remember how it was done. I was given permission and took the photo. It would not have crossed my mind to just take the picture.
The old adage that "The Customer is Always Right" is bunk. I am sorry if you believe that, but it is not true. I have dealt with verbal and sexual harassment from customers. I have been treated with incredible rudeness and had to smile my way through it. Remember when men thought it was Okay to pat the bottom of a waitress? That is not so long ago and it still happens every day.
I was so happy the day I was given permission to fire a customer and ban her from my workplace.
Yes, there may have been a misunderstanding between the Op and the store clerk. I do not know that either of them handled it well.
But the assumption of many here that, as a shopper or browser, you have the right in a private business to photograph their stock is a bit much. You may feel you have a really good reason to do so, but it is private property and stays that way until you make a purchase and take it out of the store.
At the big quilt show, I looked for signs to say if photos were allowed or not. I did not see any posted, but still asked at the information desk to make sure it was okay. Yes lots of other people were taking photos, but I was not going to unless it was explicitly allowed.
I have seen posts on this very site complaining that a person's quilt has been posted on Pinterest and that is after she posted a photo of it on the very public Internet.