The foundation of yoga is self-management. A yoga teacher cannot practice yoga in place of her student. The student has to do it herself, whether it be exercises on the mat, breathing, or meditation.
Traditionally, the methods of yoga were not intended as treatment. Yoga was intended to be a journey of self-transformation. Yet, when does healing end, and self-transformation begin?
Health is balance. Extremes in body or mind are rarely healthful. Thus the science of therapeutic yoga lies in using the skills of working with the body-mind complex to restore balance.
If I have back pain, I may not be able to sit in meditation comfortably. The skill of alignment, stabilization, strengthening, better posture in daily life, gentle release, breathing, and stress reduction, can all help to bring the function of the back to a better balance. A yoga teacher can teach this; if taught appropriately and practiced sincerely, back pain usually does reduce.
This is as much yoga as it is therapy. Perhaps, such "yoga therapy" is even more "yoga" than a pain-free individual putting his leg behind his head or dropping back into a wheel pose. After all, the goal of yoga is fundamentally to reduce suffering and improve quality of life, and that is the goal of any therapy too.
The skills of self-care are both preventive and healing. Yoga as a holistic self-management skill is valuable in both health and disease, whether we label it therapy or fitness.