Quite possibly the answer to this question, is to follow the money. It's only been in recent years that insurance has covered Chiro. That tells me that they must have realized that it works, thus saving them drug and surgery money.
Actually, it's gone the opposite direction here in Ontario. A portion of the visit used to be covered under our Ontario Health Insurance Plan. Several years ago OHIP stopped covering any portion of a chiropractic visit. The studies they did, didn't support the benifits Chiropractors were claiming.
BB