As far as I see it banks are there for you to use, not for them to use you.
I can't believe they didn't let you withdraw more than that amount, they may not even had 15k at the branch they could give away but the fact that they limit you is very alarming and I would definitely try to find a bank that wouldn't do that.
It would probably work better if you told them you are going to transfer everything you have into a different bank if they didn't give you what you wanted or that you would like to close your account and get everything out now but I guess that would also be a problem...