How can a vet spay a pregnant cat without the owners knowledge?

I took my kitten in to be spayed and found out only after it was too late that she was pregnant. I went to pick her up that afternoon and saw on the paper that pregnant was checked. They didn't even tell me. I asked if that box being checked meant that she was prego and they said yes. They weren't even going to tell me! I then asked why on earth didn't anyone call me to tell me she was pregnant before continuing and they said oh we don't do that. So, not only was I robbed of the decision, as the owner, wether to continue or not, they were not even going to tell me that she was prego. I shouldn't of had to ask them what that checked box meant. Is it even legal for them not to tell me before aborting her kittens? Isn't this considered malpractice?