Wow I understand in today's economy that most stores will sell fabric to whoever But I think it's very unfair of them to just walk up and take a bolt that your holding to cut fabric for someone else. I think they should have waited until you got the fabric you wanted cut first and then taken what was left for the other customer.
I've been in our LQS and have bolts of fabric in my hands and other customer's have tried to take a bolt I'm holding right out of my hands. I politely told the lady that as soon as I got the fabric I needed cut I would be more than be happy to give her the bolt when I was done.