In many states, the Veterinary Practice Act requires veterinarians to comply with state and federal laws, or their license to practice can be suspended or revoked. Cannabis in all forms is still illegal under federal law.
Unfortunately, this takes any medical potential of cannabis from being completely banned, to non-medical people in marijuana stores giving medical advice. If you listen to many cannabis proponents, it is the magic drug and can help or cure almost anything. It is not being explored medically in the same way normal drugs are used and tested.
Veterinarians are not in favor of retailers selling supposed medical products in lieu of established, tested, reproducible usage data and manufacturing practices. It isn’t that Marijuana is bad, but that there isn’t enough medical data about it, nor a supply of THC or CBD products that are manufactured to the standards that veterinarians expect.