I’m not sure “most” are charities, but rather private for-profit businesses. In some areas — mainly large cities — the local fire department may also offer paramedic/ambulance (land) services, but at least where I live they are separate services. Hospitals may have lifeflight helicopters, but again not most. Water rescue around here is a specific fire company with special training. Emergency medical transportation is a business and charges for the service. Sometimes insurance covers the cost.