on taking pictures in supermarkets

I was at the local grocery store today, snapping pictures of my sister and some other things like shelves, fruits, items that i found colorful and nice to take pictures of. Later, i noticed that one of the security guards was observing me. I overheard he was told to ask me to put away my camera. Apparently, i didn't know that it's not okay to take pictures in supermarkets.

Honestly, i felt offended but i made a bit of research and learned that there are some supermarkets that don't allow taking pictures because of privacy reasons or anything that might cause them to lose money because of those pictures.

But also, it never occurred to me that what i did would be anything close to spying or something like that. But i also realize now that i did take pictures of the shelves and items which showed the price tags. I decided not to post them as i don't want to get into any trouble.

I also wonder, if someone had been taking pictures using a digital camera, would they also try to call that person's attention? In my case, i already got the clue while the security guard was on his way to tell me, so i put on the cap and turned it off. He saw what i did and walked away.