Help me understand why physiatrist only wants to give trigger point injections in upper back? Have torn disc low back = spasms. Legs/arms spasm.

Trigger Points Injec. Even at times i wonder about this as well. However keep in mind that physiatrists are experts in musculoskelal issues. Trigger point injections are used for muscle pain in which a trigger point is demonstrated. I suggest you discuss this with him.
Apparent diagnosis. Apparent diagnosis of fibromyalgia or myofascial pain syndrome. He/she apparently believes "torn disk" is not cause of your symptoms.