Researching this now....I'm surprised. I see what you meant; I had no idea the transmission rates were that low.
EDIT: Looks like the rates are only low if the person has had HIV for awhile. When they first contract it, the viral load is much higher and the risk of receptive vaginal transmission jumps from 1 in 1,250 to 1 out of 50.
The demographics/statistics are interesting:
HIV in the US | HIV Statistics Center | HIV | CDC
Statistical data and information about HIV in the US.
www.cdc.gov
HIV in the United States by Region
Learn more about the impact of HIV in the US Regions.
www.cdc.gov