That is a good question, and one that I was sure to ask myself and research before attending osteopathic school. It is a fact that Americans spend more money on alternative therapy (i.e. herbal remedy, accupressure, eastern medicine, etc.) than they did on mainstream medicine. There is a patient move to more alternative treatments of disease and illness and those patients are more likely to see an osteopath. Traditionally osteopaths have been more open-minded to alternative therapy. The vast majority of M.D.'s dismiss herbs, accupressure, chiropractors, etc. as foolish. Lucky for osteopaths, we're right in the middle. We offer all the foundation and skill of mainstream medicine along with the "grab-bag" of alternative therapy. A patient in true pain would much rather be healed without the side effects of drug therapy. The move to treating the body and not just the disease (which is the entire basis of osteopathic medicine) is gaining momentum among allopathic educators. You could not pick a better time to be entering the osteopathic field.
By the way, manipulation is 100% chargeable to HMO's and insurance companies. The last I heard was that osteopaths were getting $55 for a 15 minute treatment ($220/hr, not too bad).