That's true of all doctors from oncologists to surgeons. But in this case back pain is largely caused by anxiety and inflammation, and marijuana is an anxiolytic and an anti-inflammatory, in addition to being an analgesic. So yeah, it kind of makes sense that weed would help with back pain, as it has been shown to many times over. Weed isn't a panacea by any means, but at the same time it's better than any other pharmacological option in terms of safety and efficacy.