Do some peole fake back injuries to get workers comp? How can they get away with that if a doctor has to prove it.

Fake back pain. Yes. Unfortunately people with severe back pain don't always have something that can be shown on a test like an xray or mri. And frequently an MRI may show a disc bulge which may or may not actually cause pain. The nature of the disease makes it easy for a good actor to fake it. I agree that a good way to prevent fraud is surveillance.
Yes. 31 yrs of experience says yes. There is a lot of room for interpretation in the evidence, some patients are good at expressing symptoms, and some doctors are more easily convinced than others. Video surveillance of patients should be considered in questionable cases.