Can you explain why a big box grocery store would not let me bring my own bags?

I had a backpack and a duffle bag, and was told to check them at the door with security. I was angry because I wanted a way to confirm how much I could fit in my bags, and how much I could carry.

Is that normal? Or was it that they thought that I was a thief? Has it happened to you? It’s happened at two locations in my city.