Yes. Many veterinarians utilize veterinary technicians and other staff to monitor patients under anesthesia rather than investing in expensive equipment. There is no law that states what kind of monitoring is required and it is up to each individual veterinarian to determine the monitoring that they do during surgeries.

If you have concerns then you should discuss them with your veterinarian. Most are happy to discuss the surgery protocol with you. And while no monitoring is not illegal lying about what they do is. Many will also let you view the surgery suite if you ask. Keep in mind that lower cost spay/neuter clinics are often going to be those without monitoring; it's why they can afford to do it for so cheap.