If vet is hitting your dog, I would run out of that door in a heartbeat!! I don't care if it is a light tap, I did not consent to him taking it into his own hands to correct my dog.
As for staying overnight, I believe that is a normal procedure. When my girl was spayed she stayed overnight at our vet's office. Although it is SO tough to leave them, it left me at ease knowing that during the time when the most complications arise (24 hrs after surgery) she is in the immediate presence of a qualified staff who can help her at the first sign of distress.