I don't know how it works with Vets, but I'll give my heads up with what I know.
I'm studying B. Oral Health in the Dentistry faculty, so as a future health professional, it's been dug well into my head that WRITTEN CONSENT is the most important file before any procedure proceeds. Without written consent, the patient has every right to sue the dental practitioner if there are any complications with treatment/surgery, unnecessary treatment (in this case, the removal of dewclaws), or any results which were not told to them. It is ILLEGAL to proceed any surgical procedures without informing the patient and getting their consent. The written consent is necessary medicolegally if a patient decides to sue the dental practitioner.
Anyway, what I meant was, it was illegal for the vet to proceed with any form of surgery without first informing the client. Especially if there were no health risks involved. If I decided to extract someones healthy tooth without telling them, I most likely won't be able to practice as a dental practitioner anymore.